Ted Cruz. REUTERS/Pearl Gabel Republican presidential candidate Ted Cruz spent the past week doing what was recently considered to be impossible: consolidating support from his party's establishment.

The list of people who have endorsed Cruz now includes Mitt Romney, the 2012 Republican presidential nominee; Jeb Bush, the ex-2016 GOP frontrunner; and Sen. Lindsey Graham of South Carolina.

Establishment-oriented Republicans are rushing to rally around Cruz, who is increasingly viewed as the only option Republicans have left to stop GOP frontrunner Donald Trump in his tracks.

That is a stunning development to party operatives almost a year after Cruz entered the presidential race as a pugnacious outsider. Among other things, Cruz infuriated the Washington establishment by embracing government-shutdown fights.

When discussing Cruz late last year, former President George W. Bush reportedly said he didn't "like the guy." Cruz worked on President Bush's 2000 campaign and held a position within his administration.

Graham, meanwhile, passed his support on to Cruz rather begrudgingly: He acknowledged that he thought the third remaining Republican candidate, Gov. John Kasich of Ohio, would be the best president but couldn't defeat Trump. Weeks ago, Graham joked that if Cruz were killed on the Senate floor, nobody would testify against his killer.

Romney, the figurehead of the "never Trump" movement, previously sent robocalls on behalf of Sen. Marco Rubio of Florida and Kasich. He even held an event with Kasich before the Ohio primary.

"It's an incredible circumstance. It's unfathomable, unthinkable," GOP strategist Matt Mackowiak, the founder of the Potomac Strategy Group, told Business Insider.

"He's the establishment's 16th favorite candidate," he continued, referring to the lengthy list of GOP candidates originally in the race. "Just so happens the 17th is Trump — and there's a big gap in between them."

Cruz with Donald Trump. Chip Somodevilla/Getty Images

It wasn't very long ago that Trump's reputation as a dealmaker appealed more to establishment types than Cruz's hardline, confrontational approach. In late January, Bob Dole, the former Kansas senator and 1996 GOP presidential nominee, made a point to say he wouldn't vote for Cruz and that in Congress "nobody likes him."

In contrast, Dole told The New York Times that Trump could "probably work with Congress, because he's, you know, he's got the right personality and he's kind of a dealmaker."

Gov. Terry Branstad of Iowa practically begged his constituents not to vote for Cruz in the Iowa caucuses. He called Cruz a "big oil" candidate who didn't have their energy interests in mind. He had no such message for voters about Trump.

But over the past two months, Mackowiak said, Cruz suddenly became much more palatable to a wider range of Republicans. The push is coming weeks before the crucial primary in Wisconsin, a key state in the anti-Trump effort.

"There's a real sense that a large cross section of Republicans are seeing what a disaster Trump would be," he said. "Pretty unprofessional campaign he's running, total lack of discipline. Cruz is better than people thought he'd be, and Trump is a lot worse. And Cruz has made up a lot of ground."

Polls have lent some credence to that theory. Recent surveys have shown that Cruz is bringing aboard some Republicans outside the ultra-conservative wing that has made up his staunchest support base.

When comparing the past two national Fox News polls — one released in mid-February and the other released last week — Cruz's growth in support was more than twice that of Kasich and almost four times that of Trump. The numbers suggested that Cruz was most able to benefit from other candidates' exits from the race.

"Kasich has as good of a chance as me," Mackowiak said. "You're not going to nominate someone who lost 49 states. That's why you're seeing guys ignore Kasich and go with Cruz. Endorsing Cruz doesn't mean he's their first choice — he's their choice at this point. If Lindsey Graham can get over it, anyone can."

Ted Cruz and Donald Trump. AP

Mackowiak said Cruz had started to broaden his message to appeal beyond the more extreme wings of the base. He added, however, that "it's not a natural thing for him, so I think it'll take some work."

But as has been the case for most of the primary season, the anti-Trump establishment faces a clear uphill battle.

Cruz needs to win nearly 80% of the delegates throughout the rest of the primary season to get to 1,237 delegates, the number needed to clinch the nomination. Trump, on the other hand, needs just about 50%. If neither candidate reaches that level, the fight could play out on the floor of the Republican National Convention this summer.

But the Republican primary continues to play out in unprecedented ways. The wildest part of the past week, Mackowiak said, was that establishment types were shunning the most likely nominee and instead aligning themselves with Cruz.

"The instinct is to go with the winner, because you have to be on the winning team to have some influence," he said of Trump. "He's losing support from people that like to go with winners and like to cut deals."