Former Florida governor Jeb Bush speaks as Sen. Marco Rubio (R-Fla.) looks on at the Republican presidential candidates’ debate in Boulder, Colorado on Oct. 28. (Rick Wilking/Reuters)

It started in 1998, with a $50 check out of the blue.

The money was a donation from one of the highest-placed men in Florida politics — Jeb Bush, the son of a former president who was about to be elected governor — to one of the lowest. Marco Rubio, 26, was running for the city commission in tiny West Miami.

“I remember him showing it off,” a Rubio friend recalled. “ ‘I got a check from Jeb Bush!’ ”

In the years to come, the two men formed an alliance that, at times, even looked like a politician’s odd version of friendship. Rubio, younger and gifted, provided Bush with help in the state legislature. Bush provided Rubio with donors, endorsements and — at one especially curious moment — a golden sword.

By this week, however, the relationship itself had become a kind of weapon.

The Fix's Chris Cillizza explains how Republican presidential candidate Jeb Bush missed his opportunity and opponent Marco Rubio had a strong performance in the third GOP debate. (Zoeann Murphy/The Washington Post)

“Someone has convinced you that attacking me is going to help you,” Rubio told Bush during Wednesday night’s GOP presidential debate, after Bush had criticized Rubio. The power of the comeback was in its familiarity — in Rubio’s pitying sense that he knew Bush well enough to know Bush had betrayed himself.

That moment had been coming for months, as a presidential election put the old allies on a path to collide. They schmoozed the same donors. Courted the same pro-establishment voters. Each threw insults — veiled, then not veiled — at the other.

A confrontation was coming. And people who watched these allies turn into enemies had little doubt who would win.

“It was a godsend for Marco,” a chance to show off his political talents and get out of Bush’s shadow, all in the same sentence, said Jorge Luis Lopez, a lawyer in Miami who is backing Rubio. “For years, everybody [in Rubio’s camp] always had to validate, ‘Is Marco ready to do it?’ And now, everybody sees Marco is ready to do it. And it came from the lips of his own mentor.”

For now, both Rubio and Bush are losing in this race. But both say they’re playing a longer game. After a while, both men believe, the outsiders Donald Trump and Ben Carson will fade, and voters will come looking for somebody safer. Both Bush and Rubio, of course, think the safe choice will be him.

That competition has put the campaign’s spotlight on a two-decades-long relationship that never fit conventional categories.

“Friends” was always too warm a word, even back then.

Marco Rubio, R-Coral Gables, center, the new speaker of the Florida House, greets Florida Gov. Jeb Bush, on his way to being sworn in, Tuesday, Nov. 21, 2006, in Tallahassee, Fla. (STEVE CANNON/ASSOCIATED PRESS)

“Enemies” is too strong, even now.

“Frenementor,” said Dan Gelber, a Democrat who served in the Florida House when both Rubio and Bush were in state government.

Bush and Rubio were born, 18 years apart, into vastly different American experiences. Rubio’s parents were Cuban immigrants who had worked as a bartender and a maid. Bush was a Bush. The first office he ran for was governor of Florida.

As Rubio rose in politics — interning for a congresswoman, working for a politically connected lawyer — Bush took notice. That was what the $50 was about.

Then, in 2000, the two men realized they could help each other in new and more concrete ways. Rubio was a new state legislator, at a time when term limits had cleared out the old guard. Bush was the governor, looking for a new ally.

“Jeb looked around, and suddenly Marco was one of the people he knew best in the House,” said a former colleague who worked closely with both men. Rubio advocated Bush’s ideas, and Bush steered Rubio toward conservative politics, especially the gospel of small government.

By 2005, the two men were close enough that when Rubio gave an emotional speech after winning the race to be Florida’s House speaker, Bush made a show of his mentorship. Bush honored Rubio with a gift: a sword, which he said belonged to a great “conservative warrior” named Chang.

“Chang is somebody who believes in conservative principles, believes in entrepreneurial capitalism, believes in moral values that underpin a free society,” Bush told a crowd so large that a plane had to be chartered to ferry well-wishers from Miami to Tallahassee. “Chang, this mystical warrior, has never let me down.”

This gesture was even stranger than it sounds. It appears that “Chang” was not a real person but something from a Bush family in-joke about Chinese Nationalist leader Chiang Kai-shek (“Unleash Chiang!”). Now, Jeb — whose ­father was once the U.S. envoy to Beijing — had garbled the story into something about a mystical warrior with a sword.

The sword “really meant something to Jeb,” a longtime friend and colleague of both men said. “He thought it was Marco who would continue his legacy.”

At the time, the sword seemed to mean something to Rubio, too. He hung it in a place of honor in his office — or at least, he used to.

“I have it somewhere at home,” Rubio told reporters in New Hampshire this year. “I have young kids. I don’t want them to run around with a sword.”

Bush left the governor’s office in 2007. After that, friends say, he kept up his alliance with the still-rising Rubio.

He supported Rubio, working donors behind the scenes, when Rubio took on the Florida GOP establishment in the 2010 Senate race. After Rubio won, the two would meet for coffee after workouts in the gym at the luxurious Biltmore Hotel in Coral Gables, Fla., where Bush has an office.

Then the men — who now shared friends, donors and allies — began to realize they would be rivals.

“He’s entitled to do this — there’s not something per se wrong,” Al Cardenas, a power broker in Florida politics who backs Bush, said of Rubio. “But we believe that most people would have decided not to proceed.”

Bush made clear last year that he planned to run. In April, Rubio announced his candidacy, and pointedly told his audience that America couldn’t go “back to the leaders and ideas of the past.”

This was the first of many signals: Rubio wasn’t just running alongside his old ally but against him, lumping him in with Democrat Hillary Rodham Clinton as symbols of the stale political order.

“The minute I saw [Rubio’s] announcement, I had every reason to assume, regrettably, that this was never going to be the same,” said Jorge Arrizurieta, who first met Bush in the 1980s, served in different roles during his governorship and is now a top donor.

“I’ve heard many common friends call it a betrayal by Marco, but not Jeb,” said Ana Navarro, a Republican strategist who is a friend of both men but supports Bush in this race. “Jeb’s not a guy to cry over spilled milk. Jeb’s got a goal ahead of him, and wasting time psychoanalyzing Marco’s motives won’t help him get there.”

Since then, Bush and Rubio have rarely spoken one another’s names — but they have talked about each other all the time.

Rubio, for instance, talked for 25 minutes in June at the Prescott Bush awards dinner — named for Jeb Bush’s grandfather — without mentioning the Bush name. He never mentioned Bush to a room full of seniors during a central Florida campaign stop in September.

But in both places, he implied a contrast, telling the Prescott Bush awards crowd in Connecticut that it was time to “transition from the past we are so proud of to the exciting future that awaits our country.”

Bush has also deployed a knock-him-without-naming-him strategy against Rubio. For months, he’s called for members of Congress who miss many votes — as Rubio has — to have their pay docked.

In late September, Bush tried to burnish his leadership credentials by telling a TV interviewer that he “relied on people like Marco Rubio and many others to follow my leadership” in Florida. Later, the two campaigns squabbled about which one of them had a disappointing quarter of fundraising. (They both did).

By Monday, Bush’s campaign — increasingly desperate amid a cash shortage and staff cuts — labeled Rubio a “GOP Obama” in a meeting with top donors. That may not sound like an insult, since Obama did manage to get elected president twice. But Bush meant it in the context of Republicans who view the president as inexperienced and untrustworthy.

In a broad sense, Bush was losing the argument. Rubio was overtaking him in the polls. And the very thing that had made Rubio such an attractive ally before — he shared Bush’s basic political beliefs but did a better job of selling them — made him a devastating rival now.

“I support Jeb because he’s older, he’s got a lot of experience, he was governor for two terms and did an extraordinarily good job, but if Jeb doesn’t make it I certainly hope Marco does,” said Barney Bishop, a prominent Florida lobbyist who is backing Bush but has also given to Rubio’s super PAC. “A lot of us are torn between both Jeb and Marco because we think Marco has a great future ahead of him. We don’t want to see either one of them have to do battle with each other in order to get ahead.”

Their competition finally came to a head at Wednesday night’s debate, producing the defining moment in their relationship so far.

Even on the attack, Bush seemed hindered by the relationship and by his blue-blood sense of decorum. “Could I — could I bring something up here?” he said.

“Marco, when you signed up for this [the Senate], this was a six-year term, and you should be showing up to work,” Bush said.

Rubio seemed to know that the intimacy of their relationship gave him more power, not less. He looked Bush right in the eye, knee-capped him, and then turned away from him to face the audience.

“My campaign is going to be about the future of America, it’s not going to be about attacking anyone else on this stage,” he said. “I will continue to have tremendous admiration and respect for Governor Bush.”

He was talking about his old ally as though Bush was already gone.