Online fund-raising has slowed across much of the Republican Party in recent months, an unusual pullback of small donors that has set off a mad rush among Republican political operatives to understand why — and reverse the sudden decline before it damages the party’s chances this fall.

Small-dollar donations typically increase as an election nears. But just the opposite has happened in recent months across a wide range of Republican entities, including every major party committee and former President Donald J. Trump’s political operation…

The total amount donated online fell by more than 12 percent across all federal Republican campaigns and committees in the second quarter compared with the first quarter, according to an analysis of federal records from WinRed, the main online Republican donation-processing portal…

Exacerbating the fund-raising problems for Republicans is that Mr. Trump continues to be the party’s dominant fund-raiser and yet virtually none of the tens of millions of dollars he has raised has gone toward defeating Democrats. Instead, the money has funded his political team and retribution agenda against Republicans who have crossed him.