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Currently, Trump has the best shot to reach the magic delegate number ahead of this summer's convention. Mathematically, Kasich can't clinch it before the Republican National Convention, while Cruz is a wild card.
In recent days, however, Cruz has earned some powerful -- and unlikely -- allies.
Tuesday, Wisconsin Gov. Scott Walker, once a rival of the Texas senator for the GOP nomination, threw his support behind his former foe.
Cruz has been arguing the endorsements are proof that Republicans are uniting behind him.
"Our campaign has been supported by Jeb Bush, Mitt Romney, Lindsey Graham, Mike Lee and Mark Levin. And if you want to talk about the full spectrum of the Republican party, as broad and ideologically diverse as you can imagine, that's it," Cruz said while campaigning in New York last week.
But many, if not most, of Cruz's new-found GOP fans have not always been so supportive. Here's a look:
The Wisconsin governor endorsed Cruz Tuesday ahead of his state's April 5 primary -- a crucial contest in which Trump is hoping for a substantial haul of the state's 42 delegates.
Walker praised Cruz as a "constitutional conservative" and said he was best positioned to win the Republican nomination and defeat Hillary Clinton in November.
"It is hard not to acknowledge this is a guy who has been consistent in his positions and when push comes to shove, will stand up for the people he represents over the interest in Washington," Walker said of Cruz.
But back in November 2013, Walker said in an interview on “This Week” that while Cruz is a good guy, the ideal Republican president needs to be “somebody who is viewed as being exceptionally removed from Washington.”
“I think it's got to be an outsider,” Walker told ABC News’ Jonathan Karl. “I think both the presidential, vice presidential nominees should either be a former or current governor, people who have done successful things in their states, who have taken on big reforms, who are ready to move America forward.
"So, that rules out Marco Rubio, that rules out Ted Cruz and rules out Rand Paul,” Walker said.
Former 2016 presidential candidate Jeb Bush endorsed Cruz in a statement last week, but he doesn't plan on campaigning with the Texas senator.
"Ted is a consistent, principled conservative," Bush wrote in a statement, adding, "for the sake of our party and country, we must move to overcome the divisiveness and vulgarity Donald Trump has brought into the political arena."
During the Jan. 14 Republican debate, however, Bush interrupted Cruz and Rubio's fighting, to attack Cruz's record (and Rubio's).
"This last back-and-forth between two backbench senators, you know, explains why we have the mess in Washington, D.C.," Bush said.
South Carolina Sen. Lindsey Graham's support of Cruz is perhaps the closest thing to an episode of "The Twilight Zone."
Graham told CNN that "it tells you a lot about where [Republicans] are at as a party." Graham hasn't flat-out called it an endorsement; instead he leans towards saying "he's on the Ted Train" and that he's raising money for the senator.
This is the same man who joked at the annual Washington Press Club Foundation dinner in February that if you killed Cruz on the floor of the Senate, "nobody would convict you."
As a guest on "The Daily Show" last week, Graham said, "I don't dislike Ted. Ted and I have a lot of differences."
Then he broke into laughter, admitting: "I'm getting better at this."
Graham was also asked on the show to clarify his statement in January that choosing between Trump and Cruz for president would be "like being shot or poisoned."
"Well, Donald is like being shot in the head. You might find an antidote to poisoning, but I don't know," Graham said.
Cruz recognizes the irony. He mentioned at campaign event in New York that Graham hosted a fundraiser for him recently, and said he's never had an event with someone who "three weeks ago publicly called for my murder."
Though Mitt Romney stopped short of offering a full endorsement, he said he would vote for Cruz in the Utah Republican caucuses.
"This is the time for Republicans across the spectrum to unite behind Ted," Romney said in a pro-Cruz robocall. "He is the only Republican candidate that can beat Donald Trump."
The two have a history. In 2013, Romney disagreed with Cruz's tactic, his 21-hour filibuster on the Senate floor, to try to defund Obamacare.
Cruz also failed to make Romney's shortlist of candidates he believes would make a good president. In an interview on "Meet the Press" in 2013, Romney listed Paul Ryan, Bush, Rubio and Chris Christie but when asked if Cruz stands as "a potential light of the Republican Party," Romney deflected.
"Look, I'm not going to disqualify anybody. But I think I've indicated some of the names that I think are most effective in becoming elected, and we'll see where it goes," Romney said.
On March 9, Carly Fiorina endorsed Cruz at campaign rally in Miami, Florida.
"He has fought for our religious liberty," she said. "He is a leader and reformer."
But in an interview with CNN on Jan. 3, Fiorina said that Cruz is "like any other politician" and will say whatever he needs to say to get elected.
"He says one thing in Manhattan, another in Iowa, whatever he needs to say to get elected and do as he pleases," Fiorina said. "I think the American people are tired of the political class that promises much and delivers much of the same."
She also bashed him over his involvement in the 2013 government shutdown.
"There's no honor in charging a hill that you know you can't take, only casualties, although Ted Cruz maybe got name recognition and money along the way," Fiorina said. "But President Obama wanted this shutdown. And Ted Cruz played right into his hands."
ABC News' Jordyn Phelps, Paola Chavez and Jeff Naft contributed to this report.