Gov. John Kasich and Sen. Ted Cruz propelled an already unique primary cycle further into uncharted waters on Tuesday, when they joined Donald Trump in declining to reaffirm their pledge to support their party's eventual nominee.

Experts agreed: That's unprecedented in the history of modern U.S. politics. It forebodes a nasty fight ahead at the GOP's July convention, the potential for a third-party run for the races' current front-runner and an impending rebirth of the party when this election ends and political fractures are set.

"This is unique in American political history, and it just keeps getting more and more unique," said Nancy Young, a historian of American politics at the University of Houston. "Some might even say strange."

The cycle already has drawn attention for its remarkably early start, for the largest field of Republican candidates in the modern era, for the leading candidacy of a celebrity TV star, the historic potential for a contested convention, the shouting matches in televised debates and campaign Twitter feuds.

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Divisiveness has shown up throughout the race, but until Tuesday the candidates formally had pledged their support for whoever is the eventual GOP nominee. They even signed loyalty pledges early last fall.

Neither Donald Trump, Cruz nor Kasich would renew that pledge when pressed by CNN's Anderson Cooper during a town hall event Tuesday.

"There's nothing about this that is normal," said Aaron Crawford, a fellow at Southern Methodist University's Center for Presidential History.

Trump explicitly said the pledge was off, while Cruz and Kasich declined to answer the question, saying they could not support a candidate who crossed certain lines.

The Republican National Committee said in a statement Wednesday, "The pledge is simple, each candidate agreed to run as a Republican and support the nominee."

The pledge, however, does not hold much sway, said Charles Sartain, assistant general counsel for the Republican Party of Texas and a lawyer at Gray Reed & McGraw in Dallas.

"There's no real way to enforce the loyalty pledge, and there is no way to force a candidate to support the party nominee," he said, citing the rules of the RNC.

It seems increasingly likely that the nominating race will be won at the GOP's July convention in Cleveland. Unless someone wins an outright majority in the state primaries and caucuses, the party's 2,472 delegates will re-vote on the convention floor, which could lead to the selection of someone other than the front-runner.

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Mark Jones, a political scientist at Rice University's Baker Institute, said the candidates' comments on Tuesday signaled they plan to fight for the nomination until the bitter end.

"It takes an already volatile and very tense convention setting and makes it even more so," he said.

Experts agreed a contested convention likely would favor Cruz, raising the possibility that Trump wins the most delegates but not the nomination.

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Crawford said the disavowal by Cruz and Kasich of their pledges to support the nominee had no apparent strategic underpinnings and is unlikely to give them an advantage in the convention process.

A spokeswoman for the pro-Cruz Trusted Leadership super PAC, formerly Keep the Promise I, said Trump's personal attacks on his fellow Republicans justify reluctance to affirm the loyalty pledge by Cruz and Kasich. None of the Republican campaigns responded to requests for comment Wednesday.

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For the front-runner, it is a different story. Jones said Trump is opening the possibility to run - or at least threaten to run - as an independent candidate.

"Whether or not the Republicans give him the nomination, he knows he can continue to promote himself and be successful," said Luke Macias, a Republican strategist in Texas. "I think (an independent Trump campaign) would create an unprecedented election."

Sartain said Trump would face challenges of meeting unique rules in all 50 states in order to appear on the November ballot.

Regardless of whether that happens, experts agreed the GOP will struggle to regain unity when the race is over. Trump and Cruz both built their campaigns with fiery condemnation of GOP leaders in Washington, D.C., directing Republican voter anger firmly at the Republican "establishment."

Instead of unifying the anti-establishment wing of the party, Trump and Cruz have only widened the gap between their bases. And the so-called "establishment" forces remain reluctant to rally behind either candidate.

"We're in uncharted waters for modern times," Crawford said. "Party structures will be altered by this cycle."