Democratic challengers in several key Senate races collected enormous hauls in the second quarter of the year, the latest sign of the party’s increasing strength as it tries to take control of the chamber in November’s elections.
Jaime Harrison, a Democrat who is trying to unseat Senator Lindsey Graham of South Carolina, raised more than $13.9 million in the quarter, his campaign announced on Tuesday. That total was almost double what Mr. Harrison raised in the first quarter of the year, when he outraised Mr. Graham.
“I cannot describe how grateful I am for your support,” Mr. Harrison wrote on Twitter. “I promise you this: Together, we are going to give Lindsey the fight of his political life. And we are going to win.”
Mr. Graham and several other Republicans in important races have not yet revealed their fund-raising for the full second quarter, which went from April through June.
Mr. Harrison’s total was particularly impressive, but fund-raising numbers announced by other Democratic challengers so far this month have also been strong. And they come as Republican incumbents are being imperiled by President Trump’s declining standing with voters.
Republicans currently hold 53 seats in the Senate, but their path to preserving that majority has looked increasingly shaky. If former Vice President Joseph R. Biden Jr. defeats Mr. Trump, Democrats would need a net gain of three Senate seats to take control of the chamber, since the vice president breaks a 50-50 tie. If Mr. Trump wins a second term, Democrats would need a net gain of four seats to control the Senate.
The strong numbers from Democratic Senate candidates follow a notable display of financial strength by Mr. Biden, who struggled with fund-raising earlier in his presidential campaign. Mr. Biden and the Democratic Party outraised Mr. Trump and the Republicans in June for the second straight month.
Democratic donors are not directing their money only to the most competitive Senate races, either. The race between Mr. Graham and Mr. Harrison, for instance, is not viewed as part of the top tier of competitive Senate contests. But Mr. Graham’s evolution into a vocal ally of Mr. Trump has turned him into a front-and-center target for Democrats.
Sara Gideon, who is seeking to defeat Senator Susan Collins of Maine, announced last week that she had raised $9 million in the most recent quarter. Ms. Collins is seen as one of the most vulnerable Republican incumbents.
Gov. Steve Bullock of Montana, a Democrat who is trying to unseat Senator Steve Daines, raised $7.7 million in the second quarter, and Cal Cunningham, who is challenging Senator Thom Tillis of North Carolina, brought in $7.4 million, their campaigns said.
Another Democratic candidate, Jon Ossoff, who is running against Senator David Perdue of Georgia, announced raising more than $3.45 million in the quarter. A spokeswoman for the Ossoff campaign, Miryam Lipper, noted that more than two thirds of that money came in during the last 20 days of June, after Mr. Ossoff won a competitive Democratic primary.