Almost no one is talking openly about abandoning Mr. Trump at this point. A total collapse at the top of the ticket, Republican strategists and donors agree, would only make holding the Senate harder.

But maintaining the Senate is an urgent imperative for the G.O.P.: A Democratic Senate could offer a glide path for liberal Supreme Court nominees from a President Biden, or block Mr. Trump’s judges if he won a second term. And right now, Senate Republican incumbents and candidates are losing badly in the money chase not just in the top Senate battlegrounds — states like Maine, Arizona, Colorado and North Carolina — but also in deep red states, such as Montana, where seats are now increasingly up for grabs.

Five of the most endangered Republican senators up for re-election were out-raised by a combined $18.5 million in the second quarter by their Democratic challengers, recent campaign filings show.

The private discussions about whether to shift resources toward imperiled Republican Senate candidates reflect a mix of factors: a lack of confidence that Mr. Trump will beat Joseph R. Biden Jr.; fear that the president is already a drag on down-ballot candidates; desire to maintain a G.O.P. “firewall” on Capitol Hill if Mr. Biden prevails; and the belief that money is not among Mr. Trump’s myriad problems.