Donald Trump and Ted Cruz’s mutual admiration is turning into a conservative bromance—one that could have a huge strategic payoff for the Texas senator’s presidential campaign.

Behind the scenes, Cruz has been methodologically developing Trump’s support. The two candidates have met in person at least five times and talk by phone occasionally. And in the future, they may even hold events together.

The two campaigns privately discussed a possible joint event this year in Washington, D.C., and Cruz was invited to join Trump’s infamous trip to the Mexico border, though a scheduling problem prevented him from attending, sources told The Daily Beast.

“In terms of Trump’s civility, if you’re nice to him, he’ll be nice to you. And Cruz has been nice. Cruz is playing the long game and hopes that if he survives and Trump doesn’t, the billionaire will swing to him. He’s the second choice for a lot of Trump voters,” said a source familiar with the inner workings of the Trump campaign.

“Cruz has been cultivating Donald for some time, declining to criticize him after the immigration flap or the McCain controversy, sending messages through operatives, dropping by Trump Tower when in New York,” the source continued.

The two draw from similar segments of the Republican primary electorate—sometimes, almost too similar. In the video launching his presidential bid, Cruz made a passing reference to “make America great again,” which has since become the Trump campaign’s slogan. Trump applied for a trademark on the phrase shortly after the 2012 election.

The Texas senator’s use of the phrase, even in passing, prompted the Trump Organization to reach out and warn the Cruz camp in a friendly manner not to use it again, according to a Trump campaign insider.

The relationship has proved to be mutually beneficial for the two presidential candidates, but Cruz stands to benefit the most.

If Trump’s campaign were to flame out, it stands to reason that Cruz would be near the top of the list for potential endorsements. And even without an endorsement, Cruz would likely be a strong contender for Trump supporters looking for a new candidate to support. Cruz’s PAC also received money from Trump: a $5,000 donation in 2014.

“Cruz is being very sly,” the Trump campaign insider said. “[He] attracts the same types of voter. A lot of second-choice voters for Trump are Cruz people… Cruz wants those votes.”

In Cruz, Trump has a prominent politician who is willing to defend his comments. He may even have, in his mind, a potential running mate.

After Trump said during his campaign launch that Mexican “rapists” and “killers” were migrating to the United States, Cruz stepped up to defend the comments.

“I like Donald Trump. I think he’s terrific, I think he’s brash, I think he speaks the truth,” Cruz said on Fox News.

The Trump campaign did not respond to a request for comment. The Cruz campaign declined to comment. But an aide to Cruz, speaking on the condition of anonymity, said the two had bonded over the issue of illegal immigration.

“The senator has repeatedly credited Trump for bringing so much attention to the illegal immigration issue,” the aide said. “The senator has been, in my mind, extremely clear about not wanting to belittle Trump and his supporters, because [those supporters] feel very passionately about a serious issue. That has certainly set the senator apart from the other candidates who have been critical of Trump, and by association critical of Trump’s supporters as well.”

As early as 2013, Trump believed that Cruz would run for president, a Trump campaign insider said.

The two met briefly at the Iowa Family Leadership summit in August 2013. “I thought he was a terrific guy,” Trump said on Fox News shortly after. “I met his father. Also terrific. And we had a great conversation.”

Following their first run-in, discussions began about a more formal meeting. It eventually happened in November 2013, when Cruz visited Trump Tower while on a fundraising swing through New York City.

During the meeting, the two discussed Cruz’s birthplace, Cruz told The Dallas Morning News. But Cruz’s erstwhile Canadian citizenship was not to be a problem—at least not an insurmountable one. Two months later, in January 2014, Trump donated $5,000 to Cruz’s PAC, the Jobs Growth and Freedom Fund.

Trump and Cruz met again at Trump Tower in mid-July of this year.

Just days later, Trump questioned whether Senator John McCain was a hero because he was captured as a POW, and Cruz declined to condemn the statement, calling Trump “a friend” on CNN.

Trump and Cruz have appeared at other events together, such as an American Spectator dinner and a Republican event at Trump’s Mar-a-Lago Club in Florida. And during the government shutdown in 2013, the two spoke by phone, Politico reported.

But their chummy relationship hasn’t bought Cruz immunity from Trump’s birther instincts. As recently as March, Trump cast doubt on whether Cruz could run for present, since Cruz was born in Canada.

In certain circles on the right, this is no small issue. On the day of Cruz’s presidential campaign launch, Rush Limbaugh gave a long monologue on his conservative talk radio show insisting the senator is eligible for the White House.

Trump and Cruz have not made any secret of their mutual admiration. Cruz has had no problem developing combative relationships with other Republicans in the presidential field or in Congress, but with Trump it has been nothing but kindness. If anything, their public praise understates the curiously cozy relationship that the two have in private.

— Alexa Corse contributed to this report