The Republicans are about one-third of the way toward replacing the Affordable Care Act. Why one-third and not halfway, since the House — one of two chambers in the legislative branch — narrowly approved the American Health Care Act on Thursday?

It’s not just that the bill must pass the Senate, but that senators are already saying they will want to make significant changes to the current legislation. Then, that revised bill (or whatever legislation the two chambers agree on in the “conference” process by which the two chambers resolve different versions of bills) must be approved by both bodies. In short, Thursday’s vote was almost certainly not the last Obamacare repeal vote even in the House.

Will the Senate pass something that the House can accept on a second go-round? Maybe. What the House process showed is that key constituencies in the Republican Party don’t want to be blamed for blocking one of the party’s big, long-standing goals: repealing or dramatically overhauling the Affordable Care Act. Members of the conservative Freedom Caucus, in particular, chafed at being criticized after the failure of the initial push for the AHCA in late March. President Trump refused to abandon the Obamacare repeal effort, even after other Republicans suggested that the party should move on. New Jersey’s Tom MacArthur, a member of the more moderate Tuesday Group, which was another source of opposition to the initial bill, was one of the key architects of an amendment that helped the bill finally pass. And Michigan’s Fred Upton, another Tuesday Group member, wrote another amendment that appealed to more moderate members of the House.

The GOP-controlled Senate — and each individual senator — will have similar incentives to make changes to the bill but still keep it moving toward Trump’s desk. Some senators both on the left of the Republican Senate conference (Maine’s Susan Collins) and the right (Texas’s Ted Cruz) criticized the AHCA in March. Now, though, it’s their turn, and they could be directly blamed if they are seen as standing between Trump and a law repealing Obamacare. The Republicans need at least 50 of the 52 GOP senators to back this bill, since it is going through the reconciliation process. With 50, Vice President Pence could cast the winning vote.

Still, while there will be pressure to get something done, there are several senators, groups and pitfalls that could get in the way of a Senate bill.

1. Susan Collins and Lisa Murkowski

Alaska’s Lisa Murkowski and Collins are among the four Senate Republicans who have been most likely to vote against Trump’s positions on issues, according to FiveThirtyEight’s Trump Score. (Tennessee’s Bob Corker and Kentucky’s Rand Paul are the other two.) Murkowski and Collins broke with the president on one of the closest votes of the year, the nomination of Betsy DeVos to be education secretary, forcing Pence to cast the 51st vote.

SENATOR STATE TRUMP SCORE PREDICTED SCORE PLUS/MINUS Collins ME 90% 48% +41 Paul KY 92 97 -5 Corker TN 95 95 -1 Murkowski AK 95 85 +10 McCain AZ 97 64 +33 Portman OH 97 74 +23 Young IN 97 90 +7 Gardner CO 100 44 +56 Heller NV 100 49 +51 Toomey PA 100 57 +43 Johnson WI 100 57 +43 Rubio FL 100 58 +42 Flake AZ 100 64 +37 Burr NC 100 64 +36 Tillis NC 100 64 +36 Perdue GA 100 67 +33 Isakson GA 100 70 +31 Cornyn TX 100 76 +24 Cruz TX 100 76 +24 Ernst IA 100 77 +24 Grassley IA 100 77 +24 Graham SC 100 84 +16 Scott SC 100 84 +16 Sullivan AK 100 85 +15 Cochran MS 100 89 +11 Wicker MS 100 89 +11 Hatch UT 100 89 +11 Lee UT 100 89 +11 Blunt MO 100 90 +10 Cassidy LA 100 91 +9 Kennedy LA 100 91 +9 Moran KS 100 91 +9 Daines MT 100 91 +9 Roberts KS 100 91 +9 Fischer NE 100 95 +5 Sasse NE 100 95 +5 Alexander TN 100 95 +5 Boozman AR 100 96 +4 Cotton AR 100 96 +4 Strange AL 100 96 +4 Shelby AL 100 96 +4 McConnell KY 100 97 +3 Rounds SD 100 97 +3 Thune SD 100 97 +3 Crapo ID 100 98 +2 Risch ID 100 98 +2 Hoeven ND 100 99 +1 Inhofe OK 100 99 +1 Lankford OK 100 99 +1 Capito WV 100 99 +1 Barrasso WY 100 100 0 Enzi WY 100 100 0 How often GOP senators have voted against Trump’s position Data as of May 4, 2017 Sources: ProPublica; Daily Kos; Cook Political Report; U.S. Senate; @unitedstates

Both have already criticized the AHCA. And there is a big barrier to either voting for anything like it: Both Collins and Murkowski have opposed recent GOP efforts to block Planned Parenthood from receiving federal funding. (Planned Parenthood does not get direct federal funding but is reimbursed when people on Medicaid use its services.) The AHCA has a provision barring federal funding for Planned Parenthood for a year.

If Collins and Murkowski oppose this legislation, the Republicans can’t afford any more defections.

2. The senators from Medicaid expansion states vs. the Freedom Caucus

Shelley Moore Capito of West Virginia, Bill Cassidy of Louisiana, Tom Cotton of Arkansas, Cory Gardner of Colorado, Rand Paul of Kentucky, Rob Portman of Ohio and Murkowski are among the Republican senators who come from states where Medicaid was expanded as part of the original Obamacare law. In March, Capito, Gardner, Murkowski and Portman wrote a joint letter to Senate Majority Leader Mitch McConnell complaining about how the original AHCA curtailed Medicaid. Portman, in an interview Wednesday on CNN, a day before the House vote, said that he was worried that the AHCA might cut funds for Medicaid recipients in his state who are addicted to opioids and need more medical help.

Cotton and Paul are not known as moderates and are from very Republican states. So they’ll be under some pressure to back Trump. That said, among the 50 states, Arkansas and Kentucky, have seen some of the biggest declines in their rates of uninsured in the years since Obamacare was adopted. The Congressional Budget Office has estimated that the changes in Medicaid through the AHCA would result in 14 million fewer people being on the program by 2026.

The Senate could make the bill more favorable to Medicaid recipients and ensure it gets the votes of these members. But that creates some potential tension with the Freedom Caucus in the House. Dave Brat, a Freedom Caucus member from Virginia who favored the House bill only after its latest round of changes, is already warning the Senate not to change what the House has done by “one iota.”

3. The CBO and other expert analyses of the bill

House Republicans have been criticized for voting on the AHCA before the CBO could analyze how the MacArthur and Upton amendments changed the bill and publish an updated “score” of the legislation — the CBO’s estimates of how many people will be covered, if their costs of coverage will go up or down and the overall price of the legislation. This decision by Republicans to bypass the CBO, while denying lawmakers and the public updated information, made political sense, since the March CBO report seemed to hobble the Republicans’ initial health care push.

But by the time the Senate gets around to voting on a health care bill, it’s likely that the CBO — in addition to outside groups (such as the Brookings Institution) — will issue analyses explaining exactly how the recent changes affect the AHCA. The MacArthur amendment, by giving states more flexibility to get out of Obamacare’s rules, could result in higher prices for people with pre-existing conditions and offer them less coverage. Upton pushed for additional funds for high-risk pools, which in the past have been been plagued by long waiting lists and very high premiums.

The Senate will have to deal with these findings in a way that the House avoided. These kinds of analyses could push senators to ask for more changes to the bill, to make it more favorable to the uninsured. And again, those kinds of changes could make it more complicated for the second go-round in the House.

On Thursday, just before the House vote, Avalere, a health care consulting company, released a report estimating that the high risks pools in the AHCA would cover the cost of less than a third of people with pre-existing conditions, about 600,000 of 2.2 million people.

4. Dean Heller and 2018

About 10 percent (23 of 238) of the Republicans in the House represent districts that Hillary Clinton won in 2016. Those members formed a key part of the GOP opposition to AHCA, and complicated its passage — nine of those 23 voted against it. In contrast, only three of the 52 Republican senators come from states Clinton carried in 2016: Nevada’s Dean Heller, Collins and Gardner. So 6 percent of Republican senators live in Clinton states. And among that group, only Heller is up for re-election next year.

That said, the GOP caucus in the Senate has much less room for error than Republicans in the House. If Heller joined Collins and Murkowski, his vote would kill the AHCA.

But the politics cut the other way, as well. GOP senators from red states, such as Cruz and Arizona’s Jeff Flake — both of whom were strong Trump critics in the past — may feel special pressure to back the party line on health care, since they’re up for re-election in 2018 and don’t want to encourage a primary challenge by stopping the repeal of Obamacare.

5. The “resistance”

In March, I wrote about the host of problems the AHCA faced: opposition from groups such as the AARP, everyday citizens coming to town halls slamming it, a CBO score that generated negative headlines, nearly all Democrats in Congress opposing it and a strong faction of wary Republicans, both inside the House and from influential GOP outside groups. What happened between then and now? Much of that opposition remains, with key medical groups blasting the MacArthur amendment. But the criticism of the bill subsided on the right, both from the Freedom Caucus and conservative groups such as Heritage Action, which strongly opposed the March version of this legislation but not the updated bill.

Will the right stay largely unified in pushing this bill? On the one hand, the Obamacare repeal process is now moving forward. A repeal seems possible. Groups on the right may be more reluctant to stall it now. On the other hand, the Senate changes could move the bill to the left, particularly in terms of allowing people currently on Medicaid through Obamacare to remain in the program. That could anger Heritage Action and the Freedom Caucus. And public disapproval, either in polls or expressed in town halls and calls to members, could spook members, just as it did in March.

All of these factors make it look as if getting the AHCA, or any legislation resembling it, through the Senate will be complicated. And that is true. But here’s the thing: There is now evidence, unlike in March, that Republicans really, truly want to move this bill. It is unpopular with the public. It could cause the party huge electoral problems in 2018. It is forcing Trump and Republican leaders to spend lots of time courting reluctant members. But Republicans are pushing forward despite those challenges. That is what Democrats did in 2009 and 2010 to get Obamacare signed. Republicans are not even halfway to repealing Obamacare, but their chances look much stronger than even a week ago.