"Any person in this party that does not support Trump at this point is increasing the chances of Hillary Clinton becoming president and destroying the Constitution," Franks said.
On the other side, Cruz backers say that the first-term senator should stick to his guns and withhold an endorsement -- or risk angering his own supporters who detest Trump.
"I have five rules for political endorsements," said Steve Deace, an Iowa radio talk show host who was an outspoken Cruz backer during the primary. "And Cruz endorsing Trump violates every single one of them."
Less than two months until Election Day, Trump has once again put Cruz in a bind. As Trump's poll numbers rise and now is neck-and-neck against Clinton, the GOP nominee's backers are looking for every last ounce of support to showcase that there is momentum and energy behind his campaign. If Trump falls short of Clinton by a narrow margin, Trump's backers are bound to blame holdouts like Cruz for failing to do more to help their party.
Yet if Cruz backs Trump now, he'll be accused of political opportunism and anger his core supporters who hailed his controversial speech at the Republican National Convention where he pointedly declined to endorse his party's nominee.
It would mean embracing a man who during the primaries taunted the senator with the nickname "Lyin' Ted," suggested Cruz wasn't eligible to be president due to his Canadian birthplace, and speculated that Cruz's father was involved in the JFK assassination.
Cruz risks getting little credit for backing Trump belatedly, while turning off the anti-Trump wing of the party that is bound to become more vocal -- and potentially more powerful -- if Clinton wins the White House.
"He's in a box," said one senior Senate Republican.
Yet, Trump has hired key Cruz allies, including campaign manager Kellyanne Conway and spokesman Jason Miller. And in a sign of a thawing of relations, Trump on Wednesday expressed support for a Cruz-backed, Internet-related proposal in government funding talks in the Senate.
Cruz tweeted his appreciation for Trump. But when the Senate dropped the language from the government funding bill Thursday, the Texas Republican stayed mostly quiet in the chamber -- a departure from his usually outspoken style.
In recent days, Cruz has been making comments of the GOP nominee that are emblematic of the tightrope he's trying to walk. He is declining to say if he would formally offer his endorsement of Trump, but argued that Clinton should never be elected president. In effect, he's trying to show that he is willing to help the party without formally giving his blessing to Trump.
"My focus is on defeating Hillary Clinton and helping preserve a Senate Republican majority," Cruz told CNN. "Because we need a federal government that is devoted to defending the Constitution, to defeating radical Islamic terrorism and keeping this country safe."
Asked if that meant he could endorse Trump before November, Cruz added: "I've given you my answer."
"He thinks about it every day," Cruz's former campaign Jeff Roe, who was Cruz's campaign manager, told reporters at a Bloomberg event this week.
"I'm hoping there's still time for him to come forward, or I think he and all the other people you named will be left in the rear view mirror of the Republican Party moving forward," Texas Lt. Gov. Dan Patrick, a Republican, told radio talk show host Laura Ingraham this week.
Working with fellow senators
The back-and-forth comes as Cruz is trying to showcase a more collaborate style in the Senate in recent weeks. After calling his colleagues part of a corrupt "Washington cartel," criticizing their convictions and battling them over policy and tactics for three years, he has, in the words of one of his colleagues, made "nice." He transferred $100,000 over to the National Republican Senatorial Committee recently.
And next month, he is joining the senior Texas senator, John Cornyn, to headline two fundraising events in Houston and Dallas for GOP senators up for reelection, including Missouri Sen. Roy Blunt, Iowa Sen. Chuck Grassley, North Carolina Sen. Richard Burr, New Hampshire Sen. Kelly Ayotte and Pennsylvania Sen. Pat Toomey.
"We're working on that together," Cornyn said.
Cruz himself may need help from his fellow lawmakers in two years. He faces a possible 2018 Senate primary challenge from Texas Rep. Michael McCaul, the chairman of the House Homeland Security Committee, who has increasingly made an issue of Cruz's failure to endorse Trump.
"I mean, [Cruz] pledged to support him," McCaul told Ingraham this week. "He broke his word."