Sen. Ted Cruz Rafael (Ted) Edward CruzThe Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy Murkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight MORE (R-Texas) has endorsed Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE for president, a stunning reversal after Cruz for months refused to back the GOP presidential nominee.

“Our country is in crisis. Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE is manifestly unfit to be president, and her policies would harm millions of Americans. And Donald Trump is the only thing standing in her way,” Cruz wrote in a Facebook post.

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“A year ago, I pledged to endorse the Republican nominee, and I am honoring that commitment. And if you don’t want to see a Hillary Clinton presidency, I encourage you to vote for him.”

Cruz, who dropped his own presidential bid in May, acknowledged in his post that he struggled with his decision to support Trump, harkening back to his speech at the Republican National Convention in July when he was booed off the stage for refusing to endorse the business mogul.

"In Cleveland, I urged voters, 'please, don’t stay home in November. Stand, and speak, and vote your conscience, vote for candidates up and down the ticket whom you trust to defend our freedom and to be faithful to the Constitution,’ ” Cruz said.

"After many months of careful consideration, of prayer and searching my own conscience, I have decided that on Election Day, I will vote for the Republican nominee, Donald Trump.”

In July, Cruz told the Texas delegation at the convention in Ohio that his pledge to support the eventual GOP nominee was "abrogated" when the nasty primary fight between he and Trump's campaigns became personal.

"Neither he nor his campaign has taken back a word of what they said about my family,” Cruz said as he faced backlash over his convention speech telling Republicans to "vote your conscience."

Trump and Cruz clashed heatedly as they battled for the GOP presidential nomination, with attacks often getting personal.

Trump fanned rumors that Cruz's father was involved in the assassination of John F. Kennedy, and he disparaged the Texas senator's wife Heidi in a retweet comparing an unflattering image of her to a glamor shot of his own wife Melania, a former model.

Cruz responded by calling Trump “a sniveling coward” and "a narcissist at a level I don't think this country's ever seen.” He also called Trump “amoral” and a “pathological liar.”

The Texas senator is widely viewed as having aspirations to run again for president in four years. He had been one of only three former GOP candidates to break the pledge and withhold their support for Trump, along with Ohio Gov. John Kasich and former Florida Gov. Jeb Bush.

Republican National Committee Chairman Reince Priebus warned earlier this month that there could be consequences for former GOP rivals who don't endorse their party's nominee, alluding to withholding resources for future White House bids.

Cruz on Friday described Democratic nominee Hillary Clinton as "wholly unacceptable" and said he has always been opposed to her.

He cited the Supreme Court vacancy following Justice Antonin Scalia's death in February as his primary reason to back Trump.

"We know, without a doubt, that every Clinton appointee would be a left-wing ideologue. Trump, in contrast, has promised to appoint justices 'in the mold of Scalia,' " Cruz said.

He noted that Trump earlier Friday released an expanded list of potential Supreme Court nominees that included Sen. Mike Lee Michael (Mike) Shumway LeeMcConnell shores up GOP support for coronavirus package McConnell tries to unify GOP Davis: The Hall of Shame for GOP senators who remain silent on Donald Trump MORE (R-Utah), an ally of Cruz.

A spokesperson for Lee rejected the prospect, saying it didn't change Lee's decision to withhold his own endorsement from Trump.

Cruz specifically cited Trump's opposition to a plan for the U.S. to relinquish control of functions key to the internet. Cruz is leading a GOP fight to block the White House’s push.

Cruz also mentioned the effort to repeal ObamaCare, Trump's vow to deregulate the energy industry, the businessman's pledge to revoke President Obama's executive orders on immigration and Trump's opposition to accepting "unvetted" refugees.

- Updated at 4:04 p.m.