"Hillary Clinton is manifestly unfit to be president, and her policies would harm millions of Americans", Cruz wrote Friday.
In a Facebook post Friday, Cruz says he's chose to vote for Trump "after many months of careful consideration, of prayer, and searching my own conscience".
This is a fairly shocking response from Trump seeing as how the Republican presidential nominee previously implied that Cruz's father was involved in the JFK assassination.
"I think Ted Cruz endorsing Trump at this point plays to the caricature that he is self-interested, not really principled", wrote Erick Erickson, a conservative commentator who had just published a well-received essay on why he still rejected Trump.
On Friday afternoon, Texas senator Ted Cruz did something a lot of your friends have been doing lately: He posted a long rant about the election on Facebook.
Trump accepted Cruz's support, describing it as an "endorsement" in a statement. "I hope you're defeated in your next election and if you run again in 2020 I will shout from the rooftops that you're a party-line-holding sellout". "The people will decide that".
An announcement would be the latest twist in a contentious relationship between Trump and Cruz, his closest competitor in the Republican nomination race.
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Cruz wreaked havoc during the Republican National Convention in July by telling voters "vote your conscience, "instead of endorsing the nominee".
The two candidates got on well early in the campaign.
In the last few months, Trump has hired some of Cruz's former employees, including Kellyanne Conway as Campaign Manager and Jason Miller as communications director. But Cruz is now backing a man for the nation's highest office that he considers to be a liar, a coward, and amoral. Succeeding Trump victories in New Hampshire and SC put him on a path to ultimately winning the Republican nomination.
Cruz wrestled with publicly aligning with Trump after their at-times bitter rivalry in the primaries.
After refusing to attack each other for months at the outset of the GOP primary, Trump and Cruz feuded intensely on the campaign trail when the Republican field narrowed.
In his Facebook post, Cruz listed six reasons for preferring Trump over Clinton: Supreme Court appointments, rolling back President Obama's health care plan, energy policy, immigration, national security and Internet freedom.
"If Clinton wins, we know - with 100% certainty - that she would deliver on her left-wing promises, with devastating results for our country". The vast majority of calls coming into Cruz's office had turned increasingly negative in recent weeks with many voters urging him to support Trump to prevent a Clinton victory, according to Republicans familiar with the situation. At a tense breakfast with the Texas delegation the following morning, Cruz said he wasn't going to "act like a servile puppy dog" and back Trump.