Vin Weber, a key early lieutenant in the Newt Gingrich revolution that transformed the U.S. House of Representatives and the GOP, rejected the candidacy of Donald Trump on Wednesday as "a mistake of historic proportions" for his party.

"I won't vote for Trump," said Weber, who represented Minnesota in the House for 12 years beginning in the Reagan era. "I can't imagine I'd remain a Republican if he becomes president."

Now a Washington lobbyist, Weber said he hadn't decided whether he would vote for Hillary Clinton. But he said he had no hope that Trump would change his behavior and become a suitable representative of the Republican Party.

Vin Weber Joshua Roberts | Bloomberg | Getty Images

Blasting what he called Trump's "protectionism" on trade, Weber said "the world political order, the world economic order and the Republican Party" would be "all in shambles" if the freshly crowned GOP nominee were to become president. "I think markets would collapse," he said in a telephone interview, adding that turbulence internationally could result in violent conflict.

"I don't know how the Republican Party survives if Trump becomes president," he said.

Weber is a friend and ally of House Speaker Paul Ryan, Republican Party chair Reince Priebus and Trump campaign chairman Paul Manafort. He said he sympathizes with Republicans holding public or party offices who feel they cannot distance themselves from Mr. Trump as he and some others, such as former California Republican gubernatorial candidate Meg Whitman, have done. But "I would applaud if other Republican leaders reject him."

Republican presidential nominee Donald Trump. Getty Images