Donald Trump may be on the cusp of bagging an endorsement from one of the most prominent NeverTrump holdouts, former primary rival Ted Cruz.

Sources close to the Trump campaign have confirmed Cruz is close to endorsing his former — and sometimes bitter — GOP rival. Talks between Cruz staff and the Trump camp have been regular in recent weeks, and have grown more serious as Trump has closed the gap with Hillary Clinton in key battleground states.

In recent weeks, signs began to build that Cruz was doing real harm to his own political career with his reticence to get behind Trump.

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Those with knowledge of the ongoing discussion say logistics of the endorsement are still being discussed but that it could happen as early as Monday, ahead of the first presidential debate. Those sources noted that Cruz has become seriously concerned about the long-term threats to the Constitution and conservative priorities should Democratic nominee Hillary Clinton win the White House.

The endorsement from Cruz would help as Trump closes in on complete unification of a once-deeply divided Republican Party. A Fox News poll released last week showed Trump earning the support of 80 percent of likely Republican voters. Eighty-one percent of Democrats supported Clinton in the survey.

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The endorsement may also help generate fresh enthusiasm for Trump’s candidacy among conservatives who retain any skepticism about the GOP nominee. That boost could particularly help take traditionally red states that have been flirting with competitive status, like Georgia, off the 2016 battleground table.

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Additional enthusiasm among the GOP base for Trump’s campaign could also prove disastrous for Clinton. A CBS-New York Times poll released Sept. 13 showed Trump already notching a 9-point enthusiasm advantage among his supporters over Clinton.

When the primary season started last year, Cruz and Trump formed something of an alliance, even making a joint appearance at a rally to protest President Obama’s Iran nuclear deal. But as Trump rose to the top of the polls and Cruz emerged as his main challenger, their relationship quickly soured.

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As the first-in-the-nation caucuses in Iowa approached, Trump began raising questions about whether Cruz’s Canadian birth disqualified him from the presidency, despite the opinion of most scholars that he met the definition of “natural-born citizen” because his mother was a U.S. citizen.

Hostility broke out into the open after Cruz’s victory in Iowa, with Trump accusing his rival’s campaign of spreading a false story that Ben Carson had dropped out of the race. Eventually, the phrase “Lyin’ Ted” became a staple at Trump rallies.

The race only grew more bitter from there. Trump accused Cruz of rigging the primaries because he was more adept at recruiting loyal delegates at party conventions. Trump also mercilessly used a debate slur about the real estate mogul’s “New York values” to bludgeon Cruz when the race turned to the Northeast.

The low point came when Trump insinuated that Cruz’s Cuban-born father might have been involved with the assassination of President John F. Kennedy.

Cruz earned boos from delegates at the Republican National Convention in July when he used his speech to urge Americans to vote their “conscience” — but declined to endorse Trump.

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“If he gives it, I will not accept it,” Trump retorted sharply at a press conference in Cleveland after Cruz’s speech. “I don’t want his endorsement,” he added. “Just — Ted, stay home, relax, enjoy yourself.”

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All that may soon be water under the bridge.

In recent weeks, signs began to build that Cruz was doing real harm to his own political career with his reticence to get behind Trump.

Fellow Texan George P. Bush, the state’s elected land commissioner, broke with the senator and his own family last month and backed Trump. And an August poll suggested that 43 percent of Texas Republicans wanted someone else as their 2018 Senate nominee — and that former Gov. Rick Perry might beat him if he ran.

Earlier this week on “The Laura Ingraham Show,” House Homeland Security Chairman Rep. Mike McCaul refused to rule out a primary challenge to Cruz and said there should be a “high price to be paid” by the senator, Ohio Gov. John Kasich, and other Republicans who refuse to back the GOP ticket.

Lt. Gov. Dan Patrick, who was Cruz’s campaign chairman in Texas, also called for party unity.

LifeZette Political Director Jon Conradi contributed to this report.