TAPPER: Welcome back to THE LEAD. Politics now, the biggest chance for Hillary Clinton or Donald Trump to change the course of the campaign comes Monday night. That's when the candidates will come together on stage for a 90-minute debate. With that matchup just three days away, Trump is getting a huge boost from one of its former rival. Senator Ted Cruz was booed when he declined to endorse Donald Trump at the Republican Convention after some bitterly personal attacks. But now the man who Trump nicknamed "Lyin' Ted" has changed his mind. CNN senior White House correspondent Jim Acosta joins me now with more on Cruz endorsing Trump. Jim, what changed?</s>JIM ACOSTA, CNN SENIOR WHITE HOUSE CORRESPONDENT: A lot has changed. But, mainly, it's Ted Cruz's concern about the Supreme Court, Jake. Donald Trump has lured a crucial new ally into his corner, lining up the endorsement of Ted Cruz. They fought bitterly during the primaries, as you said. With Trump at one point suggesting that Cruz's father was involved in the Kennedy assassination. Now, Cruz says he's backing the GOP nominee to keep the Supreme Court from tilting to the left, adding in a Facebook post, we'll put it up on screen, "A year ago, I pledged to endorse the Republican nominee, and now I'm honoring hat commitment. If you don't want to see a Hillary Clinton presidency, I encourage you to vote for him." No question, this announcement is a big coup for Trump at a critical time.</s>ACOSTA (voice-over): With the violence in Charlotte almost certain to loom large over their first debate, Donald Trump is previewing his lines of attack, accusing Hillary Clinton of siding against the police.</s>DONALD TRUMP (R), PRESIDENTIAL NOMINEE: Those peddling the narrative of cops as a racist force in our society -- and this is a narrative that is supported with a nod by my opponent.
Ted Cruz Flip-Flops on Trump; Ted Cruz Changes Course, Endorses Trump.