Sen. Susan Collins (R-Maine) speaks at a news conference at the Capitol in Washington on Feb. 15, 2018. Sen. Lisa Murkowski (R-Alaska) is at left. (J. Scott Applewhite/AP)

How will Susan Collins and Lisa Murkowski vote? The two U.S. senators, from Maine and Alaska, respectively, are the swing votes — moderate Republicans, women, and pro-choice — who will be under a spotlight as President Trump nominates someone to succeed Justice Anthony M. Kennedy on the Supreme Court. Liberal and conservative groups are mobilizing to lobby these and others who might stray from their party’s line. At stake is Roe v. Wade, the 1973 Supreme Court decision that protects a woman’s right to have an abortion. While campaigning for the presidency, Trump promised to nominate “pro-life” judges who would overturn or limit that ruling.

Will Collins and Murkowski vote to confirm a nominee who would do that?

A closer look at the battle to replace the last swing justice, Sandra Day O’Connor, suggests how they might vote.

Here’s how I did my research

To understand what shapes confirmation votes, I looked at President George W. Bush’s nomination of Samuel A. Alito Jr. to replace Sandra Day O’Connor in 2005, which the Senate considered in 2006. For my book Women in the Club, I examined senators’ public statements and roll-call votes about that nomination. Then, as now, senators — and many Americans — wanted to know how Alito would treat Roe v. Wade. He would replace O’Connor, the first woman on the Supreme Court, who had co-authored the majority opinion in Planned Parenthood v. Casey. That opinion upheld the right to an abortion — but did allow states to adopt restrictions so long as the regulations did not impose an “undue burden” on a woman’s ability to access abortion.

Alito did have a record that suggested his views. While on the 3rd Circuit Court of Appeals, Alito had voted on Planned Parenthood v. Casey before it reached the Supreme Court. His position was more conservative than O’Connor’s. Alone among his peers on the U.S. Court of Appeals for the 3rd Circuit, Alito thought the state should be able to require women seeking abortions to first notify their husbands. During his Senate confirmation hearings, Alito refused to call Roe v. Wade settled law. His calling it settled law would have been a signal that he was promising to uphold it. But he did say that he respected stare decisis, the idea that past decisions are binding. In the end, all but one Republican voted to confirm Alito, as did four conservative Democrats.

Alito’s confirmation offers three lessons for the upcoming confirmation contest.

More than policy shapes senators’ confirmation votes

Collins was one of only three Republican senators who voted against the ban on partial-birth abortions, a 2003 law that prohibited certain late-term abortions. But she also voted to confirm Alito, whose vote helped uphold the ban on partial-birth abortions in Gonzalez v. Carhart. Why?

Members of the president’s party are expected to support his nominees. Not doing so will alienate the president, party leaders, donors and activists in the base — which hampers a senator’s ability to pursue his or her own policy priorities.

What is more, a vote against a nominee cannot be explained away as easily as a vote on legislation. Any nominee is a person who is likely to offer a compelling personal story. That’s probably one reason that Senate Majority Leader Mitch McConnell in 2016 refused to allow President Barack Obama’s nominee to the Supreme Court, Merrick Garland, to even have a hearing.

There are many ways to avoid or explain away a vote on a proposed law. The opposition can use procedural tactics to block a vote, saving senators from going on the record on controversial measures. Even if a politician must vote, there are ways to explain that vote by pointing out problems with the bill and suggesting alternatives. Consider the attempt last year to repeal the Affordable Care Act, on which Collins and Murkowski voted no. Saying they supported the measure in principle, they opposed the bill on the floor — because it defunded Planned Parenthood and eliminated generous federal funding for state Medicaid programs.

There will be no avoiding a confirmation vote, since the president’s party controls the Senate — especially since the Senate did away with the filibuster on Supreme Court nominations. Senators can only try to influence whom the president selects as the nominee. That is why Collins and Murkowski have urged the White House to expand its list of likely nominees.

Electoral pressures can matter

Even with the strong pressure for party loyalty, senators also respond to constituent opinion, particularly the president’s political standing in each senator’s state, and the senator’s own electoral calendar. All four Democrats who voted to confirm Alito represented states that supported Bush in 2004. And three of those four knew they would face voters that fall.

What does this mean for the contest to replace Kennedy?

Conservative groups will certainly pressure Democratic senators who are up for reelection in states that  Trump won by large margins. This includes Sens. Joe Manchin III (W.Va.), Heidi Heitkamp (N.D.) and Joe Donnelly (Ind.), all of whom voted to confirm Trump’s first Supreme Court nominee, Neil M. Gorsuch.

Liberal groups will have a harder time turning Republicans against a nominee from their own party. Despite facing voters in 2006 in Democratic-leaning Maine, Olympia Snowe, a pro-choice Republican who relied on support from female voters, still supported Alito.

The only Republican to vote against Alito was Lincoln D. Chafee of Rhode Island. Chafee was in a much more competitive race with a conservative challenger in the primary and a strong Democratic opponent, Sheldon Whitehouse, who ultimately defeated him in the general election. Chafee also risked losing a coveted endorsement from the abortion rights advocacy group NARAL if he voted for Alito. But neither Collins nor Murkowski faces voters this year.

Mostly senators want to avoid blame

Collins and Murkowski know that they could vote for a nominee who may become that justice who provides the vote to overturn or gut Roe v. Wade. That’s why Murkowski emphasizes that there are other issues besides Roe before the court, and why Collins will explain the importance of precedent when she meets with nominees.

But such a court decision won’t take place for months or even years. Voters are less likely to blame a single senator for a decision made by the full court long after that senator’s confirmation vote than they are to blame a senator for her vote on legislation.

And so a Republican senator can try to avoid blame by sticking with the president’s nominee.

Michele L. Swers is a professor of American government at Georgetown University and author of Women in the Club: Gender and Policy Making in the Senate (University of Chicago Press, 2013) and The Difference Women Make: The Policy Impact of Women in Congress (University of Chicago Press, 2002).