If you’re wondering what will happen with the Senate’s effort to repeal Obamacare, I recommend you follow the words, actions and eventually the votes of four Republican senators over the next days or weeks: Susan Collins (Maine), Mike Lee (Utah), Lisa Murkowski (Alaska) and Rand Paul (Kentucky).

Why those four? First, they are part of a broader group of Republican senators who have been complaining about the GOP’s repeal-Obamacare process since the start of the year. Lee and Paul, along with Texas Sen. Ted Cruz, have generally worried that the Obamacare repeal may still leave too much of the law in place. “No Obamacare lite,” Paul wrote in February.

On the other ideological end, Collins and Murkowski — along with Sens. Bill Cassidy (Louisiana), Shelley Moore Capito (West Virginia), Cory Gardner (Colorado), Dean Heller (Nevada) and Rob Portman (Ohio) — have at times pushed back against repealing too much of Obamacare, worrying that the Republicans may not be doing enough to protect people on Medicaid or those with pre-existing conditions.

SENATOR STATE EXPANDED MEDICAID TRUMP MARGIN TRUMP SCORE Susan Collins -3 86.0% Rand Paul ✓ +30 87.8 Lisa Murkowski ✓ +15 92.9 Dean Heller ✓ -2 93.0 Ted Cruz +9 95.3 Cory Gardner ✓ -5 95.3 Mike Lee +18 95.3 Rob Portman ✓ +8 95.3 Shelley Moore Capito ✓ +42 97.6 Bill Cassidy ✓ +20 97.7 Senators who have expressed doubts about the Obamacare repeal Trump score measures how often members of Congress vote with Trump’s position on major legislation. Source: Kaiser Family Foundation

So any of these members could provide the three votes that would kill this bill. I chose those four because I would argue they would have the clearest rationales to stop an Obamacare repeal and be able to explain that vote to their Republican constituents. Based on our “Trump Score,” Collins and Murkowski are among the five Senate Republicans who most vote against Trump’s positions on major legislation. Collins has a reputation as a more moderate Republican and is in a state that Hillary Clinton carried in 2016. In 2010, Murkowski was defeated by a more conservative Republican in Alaska’s GOP primary, but then ran in the general election and won as a write-in candidate.

Also, both of these senators, unlike many of the others in the group of more moderate Republicans, have specifically complained about a provision in the bill that would effectively bar, for one year, Medicaid recipients from getting coverage at Planned Parenthood clinics. Republicans object to Planned Parenthood because it offers abortion services. (There is an existing ban on the use of federal funds to pay for abortions.)

I’m not saying other moderates won’t oppose this legislation. But if a coalition of more moderate members rises up to oppose this bill, it is very likely to include Collins and Murkowski.

On the conservative end, Cruz has been hinting that he wants to use this bill to show fellow Republicans that he can be a team player, working with the party instead of clashing with it as he did for much of 2013 and 2014. It’s hard to see him leading a lonely charge against this legislation.

In contrast, Paul in particular has been scathing. Speaking to reporters this week, before the bill’s official release, he called what he had seen of the Senate’s work on Obamacare repeal “weak-kneed.” Lee has also been a regular critic of the Senate’s bill, even before its release. And the Senate bill, like Obamacare, will use a system of tax credits that vary based on income, which makes it easier for poorer people to buy insurance. The House bill gave tax credits to people to purchase insurance based on age. That change could embolden both Paul and Lee further.

What else might affect these senators’ decisions and potentially those of their colleagues? First, how will outside groups react? Generally, groups representing patients, such as AARP, and medical groups, like the American College of Physicians, have been wary of this legislation. Will they try to aggressively mobilize their members against it? Perhaps more importantly, how will major conservative groups view the bill? One important factor in the initial failure of the House version of Obamacare repeal was that conservative groups felt it did not repeal enough of Obamacare.

Indeed, Philip Rocco, a political scientist at the Marquette University who co-wrote a book called “Obamacare Wars” that details the fights over the law in states from essentially the moment it passed, said it was particularly important to watch the moves of conservative groups such as Americans for Prosperity and Heritage Action in the context of Lee and Paul. He argued that if key parts of the conservative movement said this bill is enough of an Obamacare repeal, it will be hard for Lee and Paul to oppose it.

“I don’t think it’s right to think of them as lone wolves,” Rocco said.

Second, how will Republican House members, particularly the hard-to-please Freedom Caucus, view this legislation? Freedom Caucus members have been in contact with the Senate about the legislation, Alyssa Farah, the group’s spokesperson, told me. That said, it’s not clear if they have signed off on the final product. Republican senators will be leery of backing a bill that the Freedom Caucus does not want, since that means this legislation may not be able to pass the House.

Third, don’t forget the Congressional Budget Office. The CBO is expected to release its “score” of the bill next week. That will be a heavily covered news event. But it is unlikely to be very surprising, since this legislation is not radically different from the two House different bills that the House had evaluated by the CBO. Last month, Collins blasted the House version of this legislation, specifically citing numbers from the CBO report on it.

I have not included President Trump’s reaction as something to watch, because I’m not convinced he will be a major factor. His closed-door comments about the bill the House passed being “mean” were probably not helpful to Republicans, but it’s hard to see him publicly blasting the Senate bill or ultimately vetoing whatever legislation comes through Congress. I will acknowledge that the president’s behavior is unpredictable and that a strong denunciation of a Republican health care bill by a Republican president might shift things.

We don’t know where this process is headed. Senate Republicans say there could be a vote next week, but it could be put off if senators demand changes. A broad coalition of members could come to McConnell and say that they can’t back this bill, as what happened to the initial version. The House is still a wild card.

But this legislation will only be truly dead if it’s clear that at least three Senate Republicans are prepared to vote against it — or they actually do. And that gives a little power to this group of four senators.