Vice President Joe Biden traveled to Capitol Hill on Wednesday to convince House Democrats to support the deal, while a small group of senators were invited to the White House to get their questions answered directly from officials who sat across from the Iranians at the negotiating table. Biden meets with Senate Democrats of the Foreign Relations Committee on Thursday.
House lawmakers said Biden was candid about the strengths and weaknesses of the compromise deal. One described his behind closed doors pitch.
"I'm going to put aside my notes and talk to you from my heart because I've been in this business for 45 years," Biden said in his opening comments, according to Rep. Bill Pascrell, D-New Jersey, who attended the session.
"I'm not going to BS you. I'm going to tell you exactly what I think," the vice president reportedly said.
Obama got a boost from the leader of his party in the chamber when Minority Leader Nancy Pelosi formally announced Thursday that she was backing the deal.
SInce Republicans in the House and Senate are firmly against the Iran nuclear deal -- announced by President Barack Obama on Tuesday -- the administration is cranking up its campaign to sway concerned Democrats to back the agreement.
Under legislation that allows Congress to review the agreement, the White House needs to secure enough votes from members of his own party to sustain the President's promised veto on an resolution of disapproval -- 145 in the House and 34 in the Senate.
After the session with Biden, several House Democrats stressed that while the process is just beginning, right now the administration likely has the votes to sustain the President's veto on a resolution to block the deal.
"I'm confident they will like it when they understand it all," the vice president told reporters on his way into the session, beginning what will be a two month campaign culminating in a vote, expected in September.
Democrats, both for and against the deal, praised Biden's presentation.
"Joe Biden was as good as I've seen him," Rep. John Larson, D-Connecticut, told CNN. "I thought he did an excellent job."
Texas Democratic Rep. Henry Cuellar said Biden is a "master of detail" and helped clarify some concerns he had about the verification provisions in the deal, but he still planned to carefully study it and said he was undecided.
Pascrell also cited the verification issue as a potential sticking point but said he is leaning 'yes' on the agreement.
"On our side of the aisle there is concern and skepticism shared by a number of members but an openness to be persuaded if the facts take them that way," Rep. Gerry Connolly of Virginia said. "I think (Biden) made some real progress on behalf of the administration today."
But Democratic Rep. Steve Israel of New York, a former member of Democratic leadership, told reporters he wasn't sold yet.
"For me, I still have some very significant questions with respect to lifting of the embargo on conventional arms. And missiles. The (International Atomic Energy Agency) verification process for me is not any time anywhere, I think there are some very significant delays built into that," Israel said.
Larson noted that both Biden's presentation, along with Hillary Clinton's a day earlier, who he said spoke favorably about the deal, helped lay the groundwork for most Democrats to back the White House.
At the same time on Wednesday that the President held a news conference trying to persuade the public he had brokered an strong and effective deal with Iran, Sen. Joe Manchin, a Democrat from West Virginia, and a handful of other senators, were in a separate part of the White House meeting with some of the President's top negotiators, who had just returned from Vienna.
"I was very satisfied with an awful lot of the answers we received," Manchin told CNN.
The intimate meeting for senators was another example of the White House's effort to shore up support in the Senate where leaders believe as many as 15 Democrats could oppose the deal. If they did, it could provide Senate Republicans the votes needed to override a veto of the disapproval resolution and scuttle the deal.
But Manchin, a centrist who has close relations with senators on both sides of the aisle, said at this point he has not detected major blowback from Senate Democrats to the deal.
"At caucus yesterday I didn't get a reading there is hard, hard opposition. I did not," he said.
In fact, Manchin said he thought Republicans actually might struggle getting the 60 votes they will need to pass the disapproval resolution, much less the dozen or so votes that might be needed to sustain a veto.
One key senator whose position will be closely monitored by the White House and his colleagues from both parties on the Hill, is Sen. Chuck Schumer of New York, the third-ranking Democrats who is poised to become the Democratic leader in the next Congress. Schumer has many Jewish voters in his state who are wary of the impact of the Iran agreement on the security of Israel. Schumer said he is skeptical of the deal and won't decide whether to support it before doing his homework.
"I will sit down, I will read the agreement thoroughly and then I'm gonna speak with officials -- administration officials, people all over on all different sides," he said when asked about his decision-making process. "Look, this is a decision that shouldn't be made lightly and I am gonna just study this agreement and talk to people before I do anything else."
Sen. John McCain, R-Arizona, a leading critic of the agreement with Iran, said "the pressure will be enormous from the administration," as it tries to keep Democrats from defecting. As chairman of the Armed Services Committee, McCain said he intends to hold hearings to demonstrate what he calls the "fatal flaws" in the deal.
House conservatives speaking at a forum sponsored by the Heritage Foundation, a conservative think tank, one after another ripped the Iran deal. But they conceded that ultimately they may not be able to block it.
"The game is rigged in favor of getting this done" Ohio Rep. Jim Jordan said.