Trump had meant to name Jerry Falwell, Jr., president of Liberty University.
"We've had so many great endorsements, with Gov. [Chris] Christie and with Sarah Palin and with everybody, Vander Plaats, our friend Bob Vander Plaats from Liberty University," Trump said at a rally in Rome, New York.
A spokeswoman for Trump's campaign confirmed that he meant to say Falwell.
In fact, Trump engaged in a public sparring match with Vander Plaats
, who leads the conservative group The Family Leader, in January shortly before the Iowa caucuses.
After Trump's slip-up on Tuesday, Vander Plaats tweeted that he's still supporting Cruz.
"So sorry @realDonaldTrump, I'm "Cruzin" to the nomination with @tedcruz! No interest in "Art of the Deal." #Loyal," he wrote.
The Cruz campaign did not immediately respond to a request for comment.
Trump and Cruz have long been fighting for evangelical voters.
Early on in the primary, Trump questioned whether Cruz was an evangelical, saying "not a lot of evangelicals come out of Cuba."
Of late, Trump has slammed Cruz -- who bested Trump to win the Iowa caucuses -- as "Lyin' Ted."
"Remember Lyin' Ted Cruz, Lyin' Ted! (He) has the Bible up here," Trump said Tuesday, as he held his arm up in the air, an action Trump often does at his rallies to mock Cruz. "Puts it down, he is a liar!"