WASHINGTON — Ted Cruz retreated quickly Friday after Donald Trump warned the fast-rising Texas senator that an ­attack on him would be futile.

“Looks like @tedcruz is getting ready to attack. I am leading by so much he must. I hope so, he will fall like all others. Will be easy!” Trump wrote early Friday on Twitter.

It had been reported that Cruz, at a private fund-raiser, had questioned whether Trump has the “judgment” to serve as commander in chief. But he backtracked after The Donald’s tweet.

“The Establishment’s only hope: Trump & me in a cage match. Sorry to disappoint — @realDonaldTrump is terrific,” he replied.