But the stockpiling has prompted grumbling among some Republican strategists, who contend — mostly in private — that the cash would be better allocated to the party’s at-risk congressional candidates, many of whom are being drastically outraised by their Democratic opponents.

Mr. Trump, they argue, does not need the money now as much as the party’s congressional candidates, both because he will not face voters again for more than two years, and because he won his 2016 campaign more by relying on his megarallies and Twitter feed than on pricey campaign infrastructure.

Even some of Mr. Trump’s defenders admit that his brisk early fund-raising and spending may be more about self-preservation than about bolstering the party.

“The president is concerned about keeping his power, and part of his power is the money, and the small donors,” said Sam Nunberg, one of the first employees of Mr. Trump’s 2016 campaign. Mr. Nunberg is now a senior adviser to a political action committee started by Mr. Trump’s former chief strategist in the White House, Stephen K. Bannon, which is mounting a midterm effort urging voters to support candidates who aligned with Mr. Trump to save the president from a possible impeachment push by Democrats.

“It was important for the president to build this massive operation for his re-election to demonstrate that it would be a fool’s errand for anyone inside the party to try to primary him, and because we don’t know what’s going to happen in the House with a possible impeachment,” Mr. Nunberg said.