From April 1 to June 24, which covers most of the quarter, Ms. Gideon raised $8.1 million, far more than Ms. Collins, who raised $3 million during that time, according to filings with the Federal Election Commission. At the end of that period, Ms. Gideon had about $5.5 million on hand, compared with $5 million for Ms. Collins.
In other closely watched races, Gov. Steve Bullock of Montana, a Democrat who is trying to unseat Senator Steve Daines, raised $7.7 million in the second quarter, his campaign said. That was more than Mr. Bullock raised in the entirety of his campaign for the Democratic presidential nomination, which he ended in December.
Cal Cunningham, a Democrat who is challenging Senator Thom Tillis, Republican of North Carolina, brought in $7.4 million in the quarter, his campaign said. Jon Ossoff, a Democrat running against Senator David Perdue, Republican of Georgia, raised more than $3.45 million, according to his campaign.
Republicans sought to minimize that fund-raising energy. “Democrats will need to spend every penny to defend records that are disqualifying in the eyes of mainstream voters who will decide the outcome in key Senate races,” Jesse Hunt, communications director for the National Republican Senatorial Committee, said Tuesday.
The most endangered incumbent Democrat, Senator Doug Jones of Alabama, has not yet disclosed his fund-raising for the most recent quarter. Mr. Jones had $8.3 million in cash on hand at the beginning of April.
One of the Republicans seeking to challenge him, Tommy Tuberville, the former Auburn University football coach who has been endorsed by Mr. Trump, raised roughly $670,000 from April 1 to June 24, according to a filing with the Federal Election Commission.
That was about 50 percent more than what Jeff Sessions — the former senator and attorney general who is Mr. Tuberville’s opponent in the Republican primary runoff, set for July 14 — raised in that same period as he fought to recapture what was once his seat.