Because Republicans in the U.S. Senate changed the rules last year to prevent the filibuster of a Supreme Court nomination, President Trump can basically appoint anyone he wants to replace Justice Anthony Kennedy, who announced his retirement on Wednesday — Trump doesn’t need a single Democratic yea vote to reach a majority. Instead, the operative question is this: Will any Republican vote against Trump’s choice?

That’s a difficult question to answer before Trump makes his pick; presumably some Republicans would balk at a nominee who’s extreme enough, either ideologically or personally. But I think it’s likely that Trump will select another figure in the vein of his first Supreme Court pick, Neil Gorsuch — someone who has clear and deep conservative credentials but doesn’t have a long record of politically charged rhetoric, like (for example) publicly saying, “I will vote to strike down Roe v. Wade.” So let’s assume that kind of pick and look at the process from there.

First, let’s say all 49 Democrats oppose Trump’s pick. That’s not at all a given — remember that Democrats Joe Donnelly (Indiana), Heidi Heitkamp (North Dakota) and Joe Manchin (West Virginia) all voted for Gorsuch last year; other red-state Democrats, particularly Missouri’s Claire McCaskill, may also feel like they can’t buck Trump this close to the election. But we want to focus on the Republicans for now.

In theory, Republicans, with 51 seats, can afford one defection and let Vice President Mike Pence cast the tiebreaking vote. In practice, however, remember that Arizona Sen. John McCain, suffering from brain cancer, has not been in Washington for months. So if McCain doesn’t vote and the nomination is voted on by the 99 remaining senators, then 49 Democrats plus one Republican could stop block Trump’s nominee.

So one big question is this: If Republicans needed another vote for this new justice, would McCain, if he couldn’t make it back to Washington, resign, thereby allowing Arizona Gov. Doug Ducey, a Republican, to appoint his replacement? I would assume a Ducey pick would back Trump’s nomination.

Aside from McCain’s health, it’s worth thinking about five other Senate Republicans in particular: Maine’s Susan Collins, Tennessee’s Bob Corker, Arizona’s Jeff Flake, Alaska’s Lisa Murkowski and Nebraska’s Ben Sasse. That group, according to FiveThirtyEight’s Trump Score, represents five of the eight GOP members who have opposed the president’s position in congressional votes most often.

How often GOP senators vote with Trump’s position As of June 27, 2018 Senator State Trump Score Rand Paul Kentucky 74.6 Susan M. Collins Maine 79.7 Mike Lee Utah 82.4 John McCain Arizona 83.0 Lisa Murkowski Alaska 83.6 Jeff Flake Arizona 84.3 Bob Corker Tennessee 84.7 Ben Sasse Nebraska 87.7 Lindsey Graham South Carolina 89.2 John Kennedy Louisiana 89.2 Steve Daines Montana 89.2 Michael B. Enzi Wyoming 90.5 James E. Risch Idaho 90.5 Patrick J. Toomey Pennsylvania 91.5 Mike Crapo Idaho 91.8 Joni Ernst Iowa 91.9 Ron Johnson Wisconsin 91.9 James Lankford Oklahoma 91.9 Chuck Grassley Iowa 91.9 Cory Gardner Colorado 91.9 Ted Cruz Texas 91.9 Dean Heller Nevada 91.9 Tom Cotton Arkansas 93.2 Mike Rounds South Dakota 93.2 Deb Fischer Nebraska 93.2 John Barrasso Wyoming 93.2 Richard Burr North Carolina 94.4 Lamar Alexander Tennessee 94.4 Dan Sullivan Alaska 94.4 Jerry Moran Kansas 94.5 David Perdue Georgia 94.5 Rob Portman Ohio 94.6 Bill Cassidy Louisiana 94.6 James M. Inhofe Oklahoma 94.6 Todd Young Indiana 94.6 Johnny Isakson Georgia 94.7 Thom Tillis North Carolina 95.9 Richard C. Shelby Alabama 95.9 John Thune South Dakota 95.9 Tim Scott South Carolina 95.9 Mitch McConnell Kentucky 95.9 Shelley Moore Capito West Virginia 95.9 Roy Blunt Missouri 97.3 John Hoeven North Dakota 97.3 Pat Roberts Kansas 97.3 John Boozman Arkansas 97.3 Orrin G. Hatch Utah 97.3 Roger F. Wicker Mississippi 97.3 John Cornyn Texas 97.3 Marco Rubio Florida 97.3 Cindy Hyde-Smith Mississippi 100.0

The other three are McCain, as well as Utah’s Mike Lee and Kentucky’s Rand Paul, both of whom have, at times, opposed GOP initiatives from the right. They are very unlikely to try to block a fairly conservative Supreme Court justice. Why not? Because they are likely to line up ideologically with a conservative pick, and because they are believed to have presidential aspirations of their own and wouldn’t want to annoy the party base, which cares deeply about judicial nominations.

Let’s dispense quickly with Sasse, who has sharply criticized the president, but almost exclusively for his tone, particularly Trump’s tweets and attacks on the press. I think it’s fairly unlikely that Sasse, who also seems to have presidential ambitions, will block a conservative Supreme Court justice.

Corker and Flake are more interesting. Both are retiring at the end of this term, so have little to fear from the Republican base. Both have been sharply critical of the president in the past and not just on tone: The two senators are right now strongly attacking Trump over his tariff policies. Flake is even threatening to withhold his votes for other Trump initiatives over the tariffs, so the Supreme Court pick gives him more leverage. But here’s the thing: Corker and Flake are fairly conservative on a wide range of issues. In terms of abortion (Trump’s pick is almost guaranteed to be anti-abortion), both have traditionally opposed abortion rights and both backed an anti-Planned Parenthood provision last year.

Instead, I think Collins and Murkowski are the ones watch. They voted against that Planned Parenthood provision and have opposed a number of anti-abortion measures in Congress.

They also, of course, joined Democrats (and McCain) in killing the “skinny repeal” of Obamacare that the GOP hoped to pass last year, so they have shown the gumption to buck Trump with the world watching and a major political victory at stake. Electorally, they may have some freedom too — Collins is not up for re-election until 2020 and represents a Democratic-leaning state, while Murkowski is not on the ballot till 2022.

Ultimately, a conservative pick to replace Kennedy is a huge priority for the Republican Party — and Collins and Murkowski will be under immense pressure to back whomever Trump chooses, even if he or she is an ardent opponent of abortion rights. It’s just hard to imagine any Republicans in the Senate joining with Democrats to block Trump’s nominee, unless that person is found to have some kind of personal scandal. Such a vote would virtually guarantee that the offending GOP senator would face a primary challenge in their next election — and likely a challenge with strong support from the party base.

But I wonder if the more moderate members, either privately or publicly, are able to constrain Trump in the nomination process, pushing him to pick someone who is likely to vote like Kennedy and Roberts (meaning mostly with the conservatives but not always), instead of a justice in the mold of Samuel Alito, Clarence Thomas or Gorsuch, who are more conservative. Someone who might be hesitant to strike down Roe vs. Wade or rule in favor of a broad exemption for religious people who want to avoid offering services to lesbian, gay, bisexual or transgender Americans.

In other words, the most important fight over this nomination will likely be during Trump’s selection process, not during the confirmation hearings or the votes, when I suspect party loyalty will outweigh other concerns about the nominee, particularly for Republicans. Will the red-state Democrats and the more moderate Republicans be able to push Trump to pick a more centrist person — or will they basically be ignored in the process and dared to vote against whomever the president wants? I strongly expect the latter, both because the more centrist members of the Senate seem to have little influence in Washington and because Trump has not shown much inclination to bend to the will of the more moderate members of his party.

If the past 18 months have been any indication, expect some complaining from senators such as Collins and Flake about how partisan this nomination process is — and then for them to vote Trump’s way.