Ted Cruz says he's voting for Donald Trump for US president - a shocking about-face after he rocked the Republican convention by dramatically refusing to do so.
The Texas senator says on Facebook that he made the decision for two reasons.
First, his promise to support the Republican nominee.
And second, his belief that Democrat Hillary Clinton is "wholly unacceptable".
The flip-flop is a stunner since the Texan was booed lustily during a floor speech at his party's convention for urging Republicans to "vote your conscious" without naming Trump.
The former Celebrity Apprentice host mocking the senator's wife Heidi's looks on Twitter created a deep division between the pair, with nasty insults from both and even Trump saying he'd refuse a Cruz endorsement.
Mr Cruz finished second to Trump in a bitter primary and for months baulked in offering support, despite his previous pledge to endorse the eventual Republican nominee.
Polls have since suggested that Mr Cruz's popularity was slipping nationally and back home in Texas, where he could face a primary challenger for re-election in 2018.
Meanwhile, Ms Clinton had a four-percentage point advantage in support over Trump ahead of their first US presidential debate, according to the latest Reuters/Ipsos national tracking poll.
The September 16-22 opinion poll showed that 41 percent of likely voters supported Ms Clinton, while 37 percent supported Trump.
Ms Clinton has mostly led Trump in the poll during the 2016 campaign, though her advantage has narrowed since the end of the Democratic and Republican national conventions in July.
With just six weeks before the November 8 election, Monday's debate at Hofstra University in Hempstead, New York will be the first of three between the White House rivals.
It presents a major opportunity for them to appeal to voters who have yet to commit to a candidate after a mostly negative race in which Ms Clinton and Trump have sought to brand each other as untrustworthy and dangerous for the country.
The live, televised matchup is expected to draw a Super Bowl-sized television audience of 100 million Americans, according to some commentators.
Among those watching will be people who so far remain on the fence. This could be a sizeable group: Some 22 percent of likely voters said in the latest poll that they do not support either major-party candidate.
That was more than twice the proportion of uncommitted voters at the same point in the 2012 election between Democratic President Barack Obama and Republican Mitt Romney.
These uncommitted voters appear to be leaning more toward Trump than Ms Clinton, according to the latest poll, though they have not been convinced enough to say they will vote for him in November.
It was also possible that some of these voters would pick an alternative-party candidate like Libertarian Gary Johnson or the Green Party's Jill Stein.
Ms Clinton led a separate four-way poll that included Trump, Mr Johnson and Ms Stein. Among likely voters, 39 percent supported Ms Clinton, 37 percent favoured Trump, seven percent picked Mr Johnson and two percent supported Ms Stein.