Supreme Court Justice Anthony M. Kennedy announced his retirement Wednesday, setting up a sprint for President Trump and Senate Republicans to try to fill what some senators called the most critical court vacancy in a generation.
Mr. Trump promised to move quickly to select his nominee, and Senate Majority Leader Mitch McConnell, Kentucky Republican, said he will schedule a vote this fall with hopes of having a justice in place early in the high court’s next term.
Democrats were already sounding an obstructionist note. Minority Leader Charles E. Schumer of New York insisted that the Senate should not vote until next year, after voters choose some of the members who will decide whether to confirm the president’s pick.
Democrats and liberal activist groups were particularly worried after Mr. Trump said his nominee will come from a list of 25 names of conservative legal stars he laid out during the 2016 campaign and updated last year.
“There are certain names that are just outstanding,” Mr. Trump told reporters at the White House after meeting with Justice Kennedy, who informed the president in person of the retirement. “We’ll be looking at them, we’ll be looking at some others. But they will come from the list of 25 people.”
He said he asked Justice Kennedy for thoughts on a successor, but he declined to share what the justice said.
Conservative and liberal activists geared up for an epic fight. A conservative pressure group announced a seven-figure advertising buy Wednesday aimed at vulnerable Democratic senators.
Pro-life advocates salivated at the chance to install another justice who might vote to overturn the Roe v. Wade decision, figuring Justice Kennedy was the key stumbling block.
Liberal groups, meanwhile, said Democrats will be judged on whether they are able to stop Republicans from confirming another conservative nominee.
Hanging over the fight is the last confirmation battle, for the seat of Justice Antonin Scalia, who died in February 2016.
Republicans refused to allow a vote on President Obama’s nominee, Judge Merrick Garland, saying the vacancy should be filled by the winner of the 2016 election.
That gave Mr. Trump the chance to pick Neil M. Gorsuch, who was confirmed by a 54-45 vote only after Republicans triggered the “nuclear option” to derail a Democratic filibuster.
Democrats now say the same election-year standard should apply to the Kennedy seat.
“Millions of people are just months away from determining the senators who should vote to confirm or reject the president’s nominee, and their voices deserve to be heard now,” Mr. Schumer said. “Anything but that would be the absolute height of hypocrisy.”
Liberal groups said Mr. Schumer must win the fight — or else.
“Mitch McConnell won the showdown over the last Supreme Court vacancy. Now it’s Chuck Schumer’s turn,” said Stephanie Taylor, co-founder of the Progressive Change Campaign Committee. “Americans now and for generations will remember the strength or weakness he projects in this moment.”
Republicans laughed off suggestions of waiting until next year. They said there is a difference between a presidential vote and midterm elections, when just a third of Senate seats are in play.
“The Senate stands ready to fulfill its constitutional role by offering advice and consent on President Trump’s nominee to fill this vacancy. We will vote to confirm Justice Kennedy’s successor this fall,” Mr. McConnell said.
Based on past confirmations, the Senate has plenty of time.
The average length from a nomination to a Senate hearing is about 39 days, according to the Congressional Research Service. That would put a confirmation hearing and vote on the Senate’s calendar in September or October, well before November’s midterms.
Justice Kennedy announced his retirement on the final day of opinions for the high court’s 2017-2018 term.
He said his retirement will be effective at the end of July.
Appointed to the court by President Reagan, he has been the pivotal fifth vote on many of the biggest issues to confront the courts in the past three decades.
That raises the stakes for replacing him.
Holding the confirmation vote before the election could put pressure on Democratic senators up for re-election in states that supported Mr. Trump.
Democratic Sens. Joe Manchin III of West Virginia, Heidi Heitkamp of North Dakota and Joe Donnelly of Indiana all voted for Justice Gorsuch’s confirmation last year. Each of them is up for re-election this year, making them clear targets for Republicans to pressure.
Conservative lawmakers said they hope Justice Kennedy’s seat will be filled by a pro-life justice who will uphold laws limiting abortion.
“Now that Justice Anthony Kennedy — a 25-year defender of abortion on the Supreme Court and the key vote to perpetuate Roe v. Wade — is retiring, we urge President Trump to nominate a committed constitutionalist to the Supreme Court,” said Catherine Glenn Foster, president of Americans United for Life.
Meanwhile, liberals fear Mr. Trump’s choice will erase Roe v. Wade altogether.
“President Trump has promised to only appoint justices who will overturn Roe v. Wade. If he succeeds, Justice Kennedy’s retirement means there will no longer be enough votes on the Court to protect the constitutional right to abortion,” said Andrea Miller, president of the National Institute for Reproductive Health Action Fund.
When he was picking for the Scalia seat, Mr. Trump’s decision reportedly came down to three: Justice Gorsuch, Judge Thomas M. Hardiman and Judge William H. Pryor.
Sen. Ted Cruz, Texas Republican, said Wednesday that Mr. Trump should consider a sitting senator.
“The single best choice that President Trump could make to fill this vacancy would be Mike Lee,” Mr. Cruz told Fox News.
Mr. Lee, a Utah Republican who is on the Trump list of 25 names, told reporters he would not say no to a nomination to the high court.
⦁ Tom Howell and S.A. Miller contributed to this report.