Mr. Trump had previously announced that he and the R.N.C. had raised $207.5 million in the first month following the election. The new records show that his fund-raising fell sharply in December compared with November, with an especially notably dip after Dec. 14, the day the Electoral College formally cast its ballots to make Mr. Biden the nation’s 46th president, and reality may have set in for some of Mr. Trump’s supporters about the futility of the efforts to overturn the result.

In the two weeks leading up to the Electoral College vote, Mr. Trump and the R.N.C. had raised an average of $2.9 million every day online; in the two weeks after, the average was $1.2 million.

In fact, Mr. Trump and the R.N.C. had raised more than $2 million online every day since the election until Dec. 14. They did not raise that much again for the rest of the year, until Dec. 31, when donations spiked at the end-of-year deadline.