Updated Monday with Trump comments on Cruz.

WASHINGTON -- Donald Trump, the president Ted Cruz once labeled a "sniveling coward," "utterly amoral," a "pathological liar" and a "serial philanderer," will campaign Monday night in Houston for an erstwhile rival he delighted in calling "Lyin' Ted."

They're hardly the first political foes to trade blows and then patch things up for expedience. And ideologically, they're more natural allies than odd couple.

But the mutual disdain and nastiness reached unusual heights even for a presidential campaign, which makes their first joint rally a must-see event, even in the age of Trump.

"His father was with Lee Harvey Oswald prior to Oswald's being, you know, shot," Trump said at one point during the 2016 primaries. "Nobody even brings it up. I mean, they don't even talk about that."

Cruz warned that if Trump became president, the nation would "plunge into the abyss."

"We are looking, potentially, at the Biff Tannen presidency," he said, invoking the Back to the Future character loosely based on Trump.

It's all bygones now.

I will be doing a major rally for Senator Ted Cruz in October. I’m picking the biggest stadium in Texas we can find. As you know, Ted has my complete and total Endorsement. His opponent is a disaster for Texas - weak on Second Amendment, Crime, Borders, Military, and Vets! — Donald J. Trump (@realDonaldTrump) August 31, 2018

They've partnered on tax cuts and judicial nominations. The president has issued at least one pardon on Cruz's advice. The senator routinely plays up his access to Trump. And with El Paso Rep. Beto O'Rourke putting a scare in Republicans, it's Trump to the rescue for Cruz, at a rally Monday night at the 18,000-seat Toyota Center.

Before boarding Marine One on Monday, Trump said he has no regrets about the JFK allegation.

It was a "very, very nasty and tough campaign" and having won, he said, "I don't regret anything. ...It all worked out nicely."

As for the nickname, no regrets on that either -- but it's on the shelf.

"He's not Lyin' Ted anymore. He's Beautiful Ted. I call him Texas Ted," Trump said.

It's a remarkable chapter for a checkered bromance.

Playing nice

Before Cruz jumped into the presidential race in March 2015, he went to see Trump in New York.

Like others, he was eager for a slice of the billionaire's largesse, and a sense of whether Trump might run after years of toying with the idea. They hit it off and found much common ground, on immigration in particular.

The reason lyin' Ted Cruz has lost so much of the evangelical vote is that they are very smart and just don't tolerate liars-a big problem! — Donald J. Trump (@realDonaldTrump) March 17, 2016

Throughout the summer of 2015, Cruz and Trump kept a tacit truce. The field overflowed with establishment types and they forged an outsiders' alliance.

By November, though, with Trump the front-runner, Cruz was warning donors that he was too volatile to entrust with the nation's nuclear arsenal.

Trump hit back, calling Cruz a "bit of a maniac" whose inability to get along with fellow senators showed that he would be ineffective as president. He mocked Cruz for agreeing with him all the time, too.

These, it turns out, were mere shots across the bow. The bromance fell apart in early January at a debate in South Carolina.

Birther questions

For Cruz, there had always been the delicate matter of his birth in Canada.

Trump was the nation's most prominent "birther." For five years, he sought to delegitimize the nation's first black president, insinuating that Barack Obama's Hawaii birth certificate was fake, demanding the "long form," then refusing to accept that document's authenticity.

Cruz's birth in Calgary, Alberta, was no secret. It even turned out that he held dual U.S. and Canadian citizenship, a revelation provided to him and his parents, and the electorate, in mid-2013 by The Dallas Morning News.

Trump kept the issue in reserve for months. He said publicly that Cruz was probably eligible and could easily put the issue to rest in court. Then Cruz began pulling ahead in Iowa, and he let loose.

Why would Texans vote for "liar" Ted Cruz when he was born in Canada, lived there for 4 years-and remained a Canadian citizen until recently — Donald J. Trump (@realDonaldTrump) February 24, 2016

"A lot of people are talking about it and I know that even some states are looking at it very strongly, the fact that he was born in Canada and he has had a double passport," he said.

Cruz never had a Canadian passport.

"You have a big lawsuit over your head while you're running, and if you become the nominee, who the hell knows if you can even serve in office," Trump said at the South Carolina debate. If not for that, he'd be inclined to pick Cruz for vice president but "we can't take him along for the ride. ... It's a shame."

"Since September, the Constitution hasn't changed but the poll numbers have," Cruz said. "I recognize that Donald is dismayed that his poll numbers are falling in Iowa."

The senator had a few barbs of his own. He sneered at Trump's "New York values" and avowed support for gay marriage and abortion rights.

"Everyone understands that the values in New York City are socially liberal" and "focus around money and the media," Cruz said. And he added, "Not a lot of conservatives come out of Manhattan."

Birth of 'Lyin' Ted'

During last week's Senate debate in San Antonio, O'Rourke invoked the president's trademark epithet.

"He's dishonest. It's why the president called him `Lying Ted' and it's why the nickname stuck, because it's true," he said.

Shortly after the fights in Iowa and New Hampshire, Trump began referring to his rival as "Lying Ted," which quickly morphed into "Lyin' Ted." Trump tweeted the phrase 27 times from March 13 to May 6, 2016, and like "Little Marco" and "Crooked Hillary," it delighted his supporters.

"You are the single biggest liar," Trump said during a debate on Feb. 13, 2016. "This guy will say anything. Nasty guy. Now I know why he doesn't have one endorsement from any of his colleagues."

Wives in the middle

The feuding steadily escalated and took an especially personal turn ahead of voting in socially conservative Utah. Make America Awesome, an anti-Trump super PAC, ran an ad targeting Mormons that used a nude photo of Melania Trump, a former model, from a GQ magazine shoot more than a decade earlier.

New anti-Trump superPAC ads feature a naked Melania Trump, and a bear-skin rug... It's targeting Mormons in Utah pic.twitter.com/zUAJeooBqg — Rhys Blakely (@rhysblakely) March 21, 2016

Cruz and his campaign weren't behind the ad, but that's where Trump put the blame.

"He knew the people in the super PAC," Trump told CNN's Anderson Cooper. "I didn't send the photo to everybody in the state of Utah. He did. It was his people, who were his friends."

"He didn't send out a picture," Cooper pointed out. "It was an anti-Trump super PAC."

Trump dug in. He threatened to "spill the beans" about Cruz's wife, Heidi, if the senator wasn't careful.

It wasn't clear what he meant but he soon tweeted an unflattering photo of Heidi Cruz juxtaposed with one of the future first lady, and the comment: "A picture is worth a thousand words."

Cruz was furious.

"It's not easy to tick me off," he told reporters. "I don't get angry often. But you mess with my wife, you mess with my kids, that will do it every time. Donald Trump, you're a sniveling coward. Leave Heidi the hell alone."

JFK allegation and backlash

The peak of the nastiness came just before Trump clinched the nomination. Cruz was the only serious rival remaining.

Trump parroted a story from the National Enquirer, a supermarket tabloid run by a close friend, suggesting that Cruz's father, Rafael Cruz, had been with JFK's assassin handing out pro-Fidel Castro pamphlets in New Orleans in 1963.

The "report" was based on a grainy and highly ambiguous photo.

"[Cruz's] father was with Lee Harvey Oswald prior to Oswald being, you know, shot. I mean the whole thing is ridiculous. What is this, right, prior to his being shot? And nobody even brings it up," Trump said on Fox and Friends the morning of the Indiana primary. "What was he doing -- what was he doing with Lee Harvey Oswald shortly before the death? Before the shooting? It's horrible."

Cruz would drop out that night after losing Indiana. He wasn't going to let this slide while he still had a share of the spotlight.

He called Trump "utterly amoral," He cited a comment Trump had made years earlier on The Howard Stern Show: "He is proud of being a serial philanderer ... he describes his own battles with venereal diseases as his own personal Vietnam," Cruz said.

"This man is a pathological liar. He doesn't know the difference between truth and lies. He lies practically every word that comes out of his mouth, and in a pattern that I think is straight out of a psychology textbook, his response is to accuse everybody else of lying," Cruz told reporters. "The man cannot tell the truth, but he combines it with being a narcissist -- a narcissist at a level I don't think this country's ever seen. Donald Trump is such a narcissist that Barack Obama looks at him and goes, 'Dude, what's your problem?'"

Lingering bitterness

At the Republican National Convention in Cleveland, Cruz as runner-up got a prime time speaking slot, and refrained from voicing an explicit endorsement for Trump.

"We deserve leaders who stand for principle. Unite us all behind shared values. Cast aside anger for love," Cruz told GOP delegates and the national audience. "Every one of us has an obligation to follow our conscience."

It came across as a swipe at Trump. Conservatives were wary of Trump, whose commitment to their principles was untested and shaky, and Cruz seemed to be stirring the pot.

Delegates booed him off stage. Texas Republicans confronted Cruz the next morning, accusing him of disloyalty to the party. Cruz dug in, saying he refused to be a "servile puppy."

"I am not in the habit of supporting people who attack my wife and attack my father," he said.

Cruz eventually came around. He formally and very publicly threw his support to the GOP nominee in late September. They met during the transition. Cruz has been an ally ever since.