“We’ve come to a point where there are so many individuals with great wealth who will support even the most fringe ideas and candidates,” said Richard F. Holt, a Republican who has raised money for presidential candidates dating back to Richard Nixon.

“Now just about anybody, no matter how far out, can come up with half a million dollars,” Mr. Holt said. Party leaders and major donors now see threats that Republicans could face from obscure but well-funded candidates whose primary motivation for seeking office is that they are aggrieved over Mr. Trump’s defeat.

Geoffrey Kabaservice, a historian and the author of “Rule and Ruin,” which documents the waning influence of moderates in the Republican Party, said that while the far right had always been an important constituency for Republicans in elections, its power was usually diluted by mainstream influences. But that is much less the case today.

“The Republican Party needed those people at the grass roots so it could win,” Mr. Kabaservice said. “But it also knew it needed to keep those people under control so it could attract some moderate, business-friendly people.”