After a disastrous week of feuds and plummeting poll numbers, Republican leaders have concluded that Donald J. Trump is a threat to the party’s fortunes and have begun discussing how soon their endangered candidates should explicitly distance themselves from the presidential nominee.

For Republicans in close races, top strategists say, the issue is no longer in doubt. One House Republican has already started airing an ad vowing to stand up to Mr. Trump if he is elected president, and others are expected to press similar themes in the weeks ahead.

In the world of Republican “super PACs,” strategists are going even farther: discussing advertisements that would treat Mr. Trump’s defeat as a given and urge voters to send Republicans to Congress as a check on a Hillary Clinton White House. The discussions were described by officials familiar with the deliberations, several of whom spoke on the condition of anonymity about confidential planning…

“The conclusion has become that the guy is incorrigible,” said Thomas M. Davis III, a former House member from Virginia who is still close to many of the party’s leaders. “He’s going to leave our candidates with no choice but to go their own separate way.”