The move surprised some on Capitol Hill, given the potential of a conflict-of-interest over hiring Florida lawyer Enrique Gonzalez, a friend of Rubio's for roughly two decades.
Late in the talks, Gonzalez inserted narrowly tailored provisions in the bill that could have helped clients he had represented, including universities, cruise liners and media companies, according to a review of emails and interviews with multiple sources involved in the negotiations.
At the time, Gonzalez's firm told the media that he had made a "clean break," putting to rest questions over any conflict. Yet a few months after the bill passed the Senate in June 2013, Gonzalez returned to his old employer -- Fragomen, Del Rey, Bernsen & Loewy -- one of the nation's largest immigration firms.
Sen. Dick Durbin, an Illinois Democrat who negotiated the immigration compromise with Rubio and six other senators, said this week that Gonzalez "changed the nature of our negotiations dramatically."
"I respected his knowledge of the subject," Durbin said of Gonzalez in a CNN interview. "But I was also always struggling with his private life and his new public role in the negotiations."
The untold story of Gonzalez's role is getting new scrutiny as the presidential campaign season gets into full swing. No debate defines Rubio's time on Capitol Hill more than the immigration battle, which put him in the crosshairs of his party as he tried to emerge as a dealmaker on the emotional issue yet later abandoned the legislation.
Rubio has faced a barrage of attacks from super PACs affiliated with his rivals Jeb Bush and Ted Cruz over his role in the 2013 immigration fight. As GOP presidential candidates gather for their next debate Thursday night in South Carolina, the new behind-the-scenes details could give more fodder to his critics.
Responding to questions this week, Rubio's office said that his hire and role in the talks strictly adhered to Senate ethics rules. Gonzalez contended that he had severed all ties with the firm during the negotiations while divesting his equity in the law practice at the time, something he said went beyond what was required. A top executive at the firm sent out a memo in January 2013 saying there could be "no contacts" between the practice's lawyers and Gonzalez on policy matters. And Gonzalez said he made no arrangements to return to the practice after his stint with Rubio.
Rubio consulted ethics committee
In January 2013, as he was discussing the new position with Gonzalez, Rubio's office seemed to have recognized there could be a perception problem.
His office and Fragomen attorney Blake Chisam, who has expertise in congressional ethics matters, were in talks with the Senate Ethics Committee to ensure his hire didn't violate conflict-of-interest guidelines, according to Gonzalez. After Gonzalez took steps to break ties with the firm, Fragomen officials told their employees not to converse with Gonzalez for assistance on client matters.
Gonzalez declined to be interviewed, but he said in an email that he had no plans to return to the firm when he was negotiating the immigration deal. In a separate background document he provided to CNN, Gonzalez argued there were provisions in the immigration bill that could have also hurt some of his former and current clients. It's unclear which clients would have been impacted.
"I went to work for Sen. Rubio because I believe in his agenda," Gonzalez said. "I had severed all ties with my firm and clients, and had no agenda to pursue except the senator's."
Rubio officials defended the decision to hire Gonzalez, saying the office went "above and beyond to comply" with Senate ethics rules and the senator's own standards.
Alex Burgos, a spokesman for Rubio, said Gonzalez was hired "to get the best possible deal."
"After the bill died, Mr. Gonzalez returned to the private sector," Burgos said. "Anyone who is now trying to misconstrue his service in our office years ago is obviously trying to hurt Marco's presidential campaign."
Burgos added that Rubio demands "all of our employees comply with the spirit and the letter" of congressional ethics rules.
Gonzalez and Rubio
Yet, Gonzalez returned to his old firm very shortly after the bill passed the Senate and when there was still hope of a bicameral deal with the House.
After seeing the bill through the Senate in June 2013, Rubio began to increasingly distance himself from the bill, refusing to call on the House to pass it even as his fellow negotiators were demanding their colleagues take it up. In mid-July of that year, Gonzalez agreed to leave Rubio's office, effective that August, according to background information the attorney provided.
Gonzalez contended that he had not finalized a job offer when he left Rubio's office and had hired a legal recruiting agency to help him find work. He said he got job offers from the firm Ford Harrison and Fragomen, and later accepted a job with his old firm on September 1 -- a month after he left Rubio's office.
The Florida senator and his immigration aide appear to have known each other since at least 1998, when they both served on the West Miami City Commission. While Rubio worked his way up through the state legislature before winning his Senate seat in 2010, Gonzalez spent 17 years working at Fragomen and became a partner at the prestigious firm.
When he became Rubio's chief negotiator in January 2013, it was viewed as unusual since most senators rely on their existing staff to cut deals -- not outside attorneys.
Cruise ships and screaming matches
Still, Gonzalez was amiable, yet a powerful force in the talks, according to several people involved in the discussions. At times, though, things got heated.
In the final day of negotiations, Durbin's chief immigration aide, Joe Zogby, and Gonzalez were screaming at each other during a private session in Sen. Chuck Schumer's conference room, forcing a staffer to intervene and break them apart, according to four people with direct knowledge of the episode.
Throughout the talks, some lawmakers and staff suspected that Gonzalez was doing the bidding of his old firm, including briefing his former partners about the state-of-play of the negotiations.
According to a document provided by Gonzalez laying out his work with Rubio, the attorney said he did consult with a variety of experts during the talks but contended it was purely for informational purposes. He said he arranged a briefing for GOP staff members with lawyers in Fragomen's Australian and U.K. offices, who were experts in their respective countries' merit-based immigration systems, information that helped them shape the proposal.
The tension had been bubbling up for weeks. A day before the bill was unveiled, Gonzalez emailed staff members to the Gang of Eight with two specific requests, according to internal emails reviewed by CNN. In one provision, foreigners would be allowed to more quickly enter the United States to deal with "critical infrastructure repairs or improvements" in response to a federal or state declared emergency.
In another provision, Gonzalez wrote in language that would let cruise line operators allow foreign workers to enter the United States to make repairs on ships for up to 90 days, circumventing the cumbersome and costly visa process. Proponents argued that the provision was necessary to attract foreigners who were experts in the technicalities of ship repairs.
Both narrowly tailored provisions would have helped specific industries in the hurricane-prone state of Florida. But they also would have been beneficial for clients Gonzalez had represented.
"It would have been high impact for a very small number of companies," said another person involved in the talks, who asked to remain anonymous to speak candidly.
Records showed that Gonzalez had represented Florida Power & Light, an electric utility in the Sunshine State that could have benefited from the provision allowing foreign workers to help repair critical infrastructure.
Moreover, the website of Gonzalez's firm touts that the attorney has worked with federal officials on behalf of "several cruise lines" to help admit foreign workers for dry dock work. In addition, it touts his work to help "several" cruise ships authorize 4,000 temporary employees during the 2005 Super Bowl in Jacksonville, Florida. His clients included Carnival, a major cruise line operator.
While several senators got carveouts for home-state interests, including Schumer's help with visas for Irish immigrants and Colorado Sen. Michael Bennet's aid for the ski industry, negotiators said that the cruise ship provision seemed unusual given Gonzalez's ties to the industry.
Durbin said this week: "It doesn't surprise me at all" that Gonzalez represented Carnival.
Allies of Gonzalez argue that it's a non-issue since he was not representing those clients at the time of the talks and had no arrangements to do so later.
Chisam, the partner at Fragomen, said: "At all times before, during and after Mr. Gonzalez's departure from Fragomen, and his employment with the office of Sen. Rubio, he and our firm complied with the letter and spirit of all Senate Ethics Committee requirements and restrictions."
There were other provisions, too, that could have benefited Gonzalez's firm. Fragomen is heavily involved in H-1B visa applications for high-skilled workers, and Gonzalez played a lead role negotiating the bill's proposal to expand legal immigration for those workers.
Moreover, Gonzalez pushed for language that said certain media, entertainment and academic personnel could receive honoraria under the law. Gonzalez represented universities along with media companies, including Bloomberg and Viacom. Sources involved in the talks said that Gonzalez pushed to include language to make it easier for multinational executives and managers to stay in the United States during periods of their work.
Universities, which Gonzalez had represented, were among the biggest beneficiaries of the bill since graduates with advanced degrees in scientific fields would be exempt from worldwide visa caps.
At the Judiciary Committee markup in May 2013, Sen. Chuck Grassley, R-Iowa, raised concerns about what he viewed as industries receiving "special treatment" under the bill.
"It appears to be a special provision for higher educational institutions and outlets," Grassley said, referring to the honoraria measure.
Schumer defended the provision on behalf of Rubio, who does not sit on the Senate Judiciary Committee, saying that that the measure was designed to help foreign "entertainers who have to come here for concert shows."
Grassley continued to raise concerns about the provision to allow foreign workers to repair cruise ships.
"It seems to me like we would already have in these countries Americans that could perform this work that you wouldn't have to have this special provision," Grassley contended.
Schumer responded: "This one, I understand, was requested by the cruise ship industry when a ship is built abroad and it has -- emergency need to be repaired. They needed to bring in foreign workers. They are temporary to fix that ship such as with the -- that Carnival ship that had all that trouble."