Former House Speaker Newt Gingrich warned in a private meeting that Donald Trump may put the GOP on course for a crushing defeat on Election Day, according to audio obtained by ProPublica.

Gingrich, a contender to serve as Trump's running mate, reportedly made the remarks at the Republican State Leadership Committee in February. His candid analysis at the Washington event seems markedly different from his recent public comments.

If Trump did not pivot toward the general election, Gingrich argued, Trump appeared poised for a defeat like Barry Goldwater in 1964.

"I had a very sophisticated medical doctor in Des Moines write me two days ago and say he sent a thousand dollar check to Trump. And I wrote back and said, 'What are you doing?'" Gingrich said to laughs.

Gingrich added that "Donald Trump's not a conservative," but "an American nationalist" who dumbs down his rhetoric for the American people.

"This is not a guy who's shallow or simple, but he is a guy who knows an immense amount about marketing, which is why he talks at a fourth-grade level," Gingrich said. "He talks at the lowest level of any candidate in either party, not because he's stupid. He does it because he knows if you talk at a fourth-grade level everybody can understand you."

Gingrich also said at the time he suspected Trump's team would "evolve rapidly" if they recognized the problems that they could confront on Election Day.

Trump has said he will announce his vice presidential pick on Friday morning in New York City.