Senator Kelly Loeffler, a Georgia Republican in a tough fight to win the seat to which she was appointed in 2019, raced after Judge Ruth Bader Ginsburg’s death on Friday to declare that she would back President Trump in filling the Supreme Court seat just weeks before Americans went to the polls. And she wanted voters to know that she didn’t hesitate.
“As the first U.S. senator to call for a nomination, I look forward to supporting a strict constructionist who will protect the right to life, defend the Second Amendment, fight for religious freedom and safeguard our values,” Ms. Loeffler wrote in a fund-raising message sent to her supporters on Friday night.
Ms. Loeffler might have been the first endangered Republican to embrace Mr. Trump’s nomination in the hopes that it could bolster a struggling campaign — but she certainly wasn’t the last. Senators Martha McSally of Arizona and Thom Tillis of North Carolina quickly followed suit, in an indication of just how profoundly the coming Supreme Court confirmation fight could scramble an already intense battle for control of the Senate.
The intensifying struggle over replacing Justice Ginsburg has thrown a volatile new element into the already intense battle for the Senate, increasing uncertainty about the outcome. Though it is hard to predict exactly how the fight will play out in the dozen or so races that will determine who holds the Senate majority, both sides are intent on using the process to their advantage.
While much attention is focused on the presidential contest, it is conceivable that the Supreme Court showdown could have a bigger effect on individual Senate races, given the direct role that senators on the ballot will play in deciding what happens — and when.
As they assessed the landscape, Republican strategists said they viewed the Supreme Court conflict as a benefit to their side. One top party adviser conceded that it could prove problematic for candidates like Senators Cory Gardner of Colorado and Susan Collins of Maine, both running in states where Mr. Trump is a drag on their chances.
But for Republican senators like Mr. Tillis and Joni Ernst of Iowa, lagging in states where the president prevailed four years ago, the adviser said the debate provided a valuable chance to emphasize their allegiance to Mr. Trump and underscore the stakes of the election. Those more Republican-leaning states will likely be decisive in determining control of the Senate.
Republicans also point to 2018, when the toxic battle over the confirmation of Justice Brett M. Kavanaugh energized their party’s voters who turned out Democratic incumbents in conservative Indiana, Missouri and North Dakota. Top party strategists said the recent electoral history suggests that a Supreme Court fight would be a political winner for Republicans, particularly given that many of the contested Senate races are playing out in states Mr. Trump won.
But Democrats see a clear advantage for their candidates. They noted that the court opening has already driven up fund-raising, and argued it would motivate voters who would be incensed at what they saw as a hypocritical double standard being applied by Republicans, who in 2016 blocked President Barack Obama’s Supreme Court nominee, saying the next president should choose the next justice.
Democrats also said it would allow their candidates to emphasize the threat to the Affordable Care Act posed by a reconfigured court, which previously upheld the law by one vote. Preserving the law was a winning issue for Democrats in 2018, and it is a major party campaign theme again this year.
Democrats have also hit Republicans for quickly pledging to confirm Mr. Trump’s pick even without knowing the nominee’s identity, saying the reactions underscored their unquestioning loyalty to the president, which they argued would alienate many voters.
“Republicans have been dragged down all cycle by their records of refusing to stand up to the president for what’s right and voting to gut health care protections for pre-existing conditions,” said Lauren Passalacqua, a spokeswoman for the Democratic Senatorial Campaign Committee. “The question of filling this vacancy shines an even brighter spotlight on these vulnerabilities.”
Democratic contenders have all called on the Senate to delay any confirmation until after the next president’s inauguration Jan. 20.
“When it comes to making a lifetime appointment to the Supreme Court, Washington shouldn’t rush that process for political purposes,” said Mark Kelly, the former astronaut challenging Ms. McSally in Arizona.
Ms. Collins, too, urged a pause until after the election, saying that whoever wins on Nov. 3 should nominate the next justice. Her position suggested that Ms. Collins, in the fight of her career for re-election, could find the confirmation fight to be a saving grace, using it to try to showcase independence from Mr. Trump and restore her moderate bona fides that were tarnished after she supported Justice Kavanaugh in 2018.
But Nathan Gonzales, the editor of the nonpartisan newsletter Inside Elections, said he was skeptical that that position would help the Maine Republican, whom recent polls have shown lagging her Democratic challenger, Sara Gideon.
“Even if Collins can delay announcing a decision on Trump’s choice, the situation reminds Democratic voters about her Kavanaugh vote,” said Mr. Gonzales, who described that as the “polarizing issue that makes her as vulnerable as she is.”
The fight over the vacancy is also likely to elevate as campaign issues Democratic threats to eliminate the Senate filibuster and add seats to the Supreme Court — issues that Republicans believe alienate moderate and independent voters. Several Democrats have suggested that their party would retaliate if Republicans proceeded with the confirmation, a prospect that thrills their core supporters.
“This puts Democrat candidates in the uncomfortable position of having to disclose to voters they are controlled entirely by Chuck Schumer and the insurgent wing of liberal extremists that have hijacked the Democratic Party,” said Jesse Hunt, a spokesman for the National Republican Senatorial Committee, referring to the Senate Democratic leader. “What had been a poorly kept secret will now be thrust into open view, hurting Democrats’ chances to appeal to independent voters in key Senate races.”