WASHINGTON — Narrowly outnumbered in the Senate, Democrats are embarking on a Hail Mary campaign to block President Donald Trump’s pick for the U.S. Supreme Court.

And they realize what a long shot it is.

“Let’s be clear: Democrats have no way to hold up this confirmation or delay the process. Mitch McConnell can and will hold a vote on Trump’s pick,” Indivisible — a liberal advocacy group launched after Trump’s election — said on its website.

Flipping a moderate Republican is probably their only hope. And that only works if they can keep Democrats who represent red states that Trump won from breaking ranks.

Before Trump’s expected announcement today, Democrats launched a three-pronged approach to defeating his nominee: Frame the process around women’s health and the future of legalized abortion, mobilize the kind of nationwide protests that helped kill Republican efforts to repeal the Affordable Care Act, and focus their attention on five key senators whose votes will determine whether Trump’s pick is confirmed.

Unlike the nomination of Neil M. Gorsuch last year to replace the reliably conservative Antonin Scalia, Trump’s newest nominee will replace Justice Anthony M. Kennedy, who was a swing vote on many of the most divisive social issues, including the landmark abortion ruling Roe v. Wade.

That makes this confirmation “more consequential than Gorsuch,” said Jeb Barnes, a political science professor at the University of Southern California. “It’s not just who you appoint, but who you replace. We know everyone who is on Trump’s list is to the right of Kennedy, so it is going to edge the court right.”

Democrats are zeroing in on Trump’s repeated promises to only appoint justices who would overturn Roe.

Sen. Dianne Feinstein of California, the highest-ranking Democrat on the Judiciary Committee, has said that “the effect of this Supreme Court nomination on women’s rights can’t be understated.” In Twitter posts and essays, Senate Minority Leader Charles E. Schumer (D-N.Y.) has echoed those concerns.

Democrats are pouncing on Trump’s use of a list of 25 potential candidates drafted by the conservative Federalist Society, which vetted all the names to ensure that they were committed to a conservative ideology.

“While these litmus-test-style commitments may have been politically sensible for Donald Trump at the time when he was running in the campaign in 2016, we believe they will come back to haunt his nominee in this summer’s confirmation battle,” said Brian Fallon, Hillary Clinton’s former press secretary, who now runs the liberal advocacy group Demand Justice.

Prominent activist groups like Demand Justice, Indivisible, MoveOn and the Progressive Change Campaign Committee are already planning a week of protests at home-state offices of senators across the country after Trump announces his nominee.

The groups are following the same grass-roots strategy that helped kill the Republican attempt to repeal the Affordable Care Act last year.

Planned Parenthood’s activism arm has held events across the country to attract local news coverage.

And the groups are flooding the airwaves with ads, urging Americans to call their senators, hoping the outpouring will jam phone lines on Capitol Hill and sway senators.

“It is game time; this is not a drill,” says an ad from NARAL, an abortion-rights nonprofit.

Demand Justice plans to spend $5 million in states with senators who might flip, with radio, television and online ads opposing Trump’s nominee. The group was recently formed as a counterweight to Judicial Crisis Network, a right-leaning group whose advocacy helped block President Barack Obama’s nominee from being considered in 2016 and supported Gorsuch’s nomination in 2017. The conservative group has also pledged to spend at least a million dollars for ads encouraging people not to believe Democrats on Kennedy’s replacement.

Barnes said the protests seem to be more about convincing liberal activists that Democrats are putting up a solid effort, rather than actually stopping Trump’s choice from being confirmed.

“They are going to have to do something to appease the base,” Barnes said. “You can only play the hand you’re dealt with. They have a weak hand.”

With Sen. John McCain, R-Ariz., receiving cancer treatment at home, Democrats have to persuade just one Republican to vote no to defeat Trump’s nominee, as long as they ensure that no Democrats vote yes.

Two Republican senators thought to be the most likely to flip, Susan Collins of Maine and Lisa Murkowski of Alaska, are the targets of television ads in their districts and in Washington, urging them to vote no. The two Republican women helped defeat the bill to repeal the Affordable Care Act last year, and both have reservations about how the potential nominees on Trump’s list might rule if Roe v. Wade is challenged.

Collins has said she wants assurance that any potential justice will treat the precedent in Roe v. Wade as settled law.

“A candidate for this important position who would overturn Roe v. Wade would not be acceptable to me, because that would indicate an activist agenda that I don’t want to see a judge have,” Collins said on ABC’s “This Week.” “What I want to see is a nominee who, regardless of his or her personal views on the very difficult and contentious life issue, is going to respect precedent.”

Murkowski, who also supports access to abortion, has been more circumspect in her public comments since Kennedy’s retirement, telling the Washington Post that Roe v. Wade’s future is a “significant factor” in her decision, but not the only one.

“And I don’t think it should be the only factor for anybody,” she said. “It’s not as if those are the only matters that come before the Supreme Court.”

Persuading a Republican to vote no won’t help if any of the three red-state Democrats who voted to confirm Gorsuch, Trump’s last pick for the court, vote with Republicans. All three face tough re-election fights in November in states Trump won by huge margins and can expect political blowback regardless of how they vote.

Sens. Joe Manchin III, D-W.Va., and Joe Donnelly, D-Ind., oppose abortion rights. And Sen. Heidi Heitkamp, D-N.D., is a bit of a maverick who has been willing to vote with Republicans in the past.

If Democrats can keep the Supreme Court debate about the future of the Affordable Care Act, one of the few things that have united every Democratic member since Trump was elected, then Schumer could have a chance of keeping them together.

The president quickly framed the confirmation vote as a campaign issue, and painted Heitkamp as automatically following Democrats’ party line.

“Heidi will vote ‘no’ on any pick we make,” Trump said at a rally last month in her home state. “She’ll be told to.”

But then all three Democrats who voted to approve Gorsuch were courted by Trump at a White House meeting the next day. They have not said how they will vote.