By JOE WILLIAMS and RYAN KELLY

Senate Majority Leader Mitch McConnell’s 13-member health care working group has gotten a lot of attention. But in the Senate, where a minority group of members can effectively stall any legislation from advancing, buy-in from the broader Republican Conference will be necessary for the GOP to succeed in overhauling President Barack Obama’s signature domestic policy achievement, the 2010 health care law.

Given those dynamics, Roll Call has compiled its own roster of 13 lawmakers to watch as debate on the repeal bill continues. The list represents the various factions within the Republican Conference and the opposing views on key issues.

The GOP is employing the fast-track budget mechanism known as reconciliation to change the law. The process requires only a simple majority of members to support the legislation, as opposed to the standard 60-vote threshold.

This only amplifies the voice of each of the 52 Republican senators. Since the GOP can only afford to lose up to two votes in the event of a tie-breaking vote from Vice President Mike Pence, the margin for victory is very small.

Democrats up for re-election in 2018 in states with large Medicaid populations or where consumers have seen significant premium increases under the current health care system were also included.

While the GOP is expected to continue to try to pass a bill with only Republican support, the Democrats on the list have made initial, albeit small, attempts at bipartisan work on the improvements to the current system and are worth watching as the health care debate continues.

For moderate senators, Medicaid is the name of the game.

Thirty-one states plus the District of Columbia have expanded their Medicaid programs under the 2010 health care law. The legislation to overhaul the U.S. health care system passed by the House last month would make significant changes to the entitlement program. That has put several lawmakers in a politically difficult situation. It is one of the biggest hurdles Senate Republican leadership faces in trying to find the 51 votes needed to advance a bill.

Aside from a Medicaid overhaul, moderate senators have also expressed concerns over the proposed restructuring of the federal subsidies to help consumers afford insurance and another provision that would affect federal funding for Planned Parenthood.

Sen. Rob Portman, R-Ohio

Portman has emerged as a key voice in the Medicaid debate. His state adopted the expansion and Medicaid now covers roughly 25 percent of the population of Ohio. It has also become a crucial lifeline for many in his state who are suffering from opioid addictions. Portman is leading a smaller working group of members from states that expanded Medicaid under the 2010 law. The group advocates a more gradual phaseout of the law’s Medicaid expansion.

Sen. Bill Cassidy, R-La.

Cassidy, a physician by training, is one of the chamber’s foremost experts on health care policy. While he has voiced concerns over a number of areas of the House bill, there is one glaring concern from back home. Louisiana has one of the largest Medicaid populations in the country. The program covers just over 30 percent of the state’s population.

Sen. Susan Collins, R-Maine

Collins, the marquee moderate Republican in the Senate, has been very vocal in her disapproval of the House plan. She has lambasted the bill’s restructuring of the health care law’s subsidies, which the nonpartisan Congressional Budget Office said could increase rates for older Americans. As one of two GOP senators to vote against a 2015 bill to repeal the health care law that was later vetoed by Obama, expect Collins to drive a hard bargain for her support.

Sen. Lisa Murkowski, R-Alaska

Alaska faces some of the highest health care costs in the nation. Its small, rural population lacks widespread access to both providers and insurance companies. Alaska also expanded Medicaid under the health care law and the program covers roughly 20 percent of the state’s population. Expect all of that to factor into Murkowski’s stance on any legislation that would overhaul the current health care system.

Sen. Jeff Flake, R-Ariz.

One of two Republican senators considered vulnerable in next year’s midterms, Flake is in a difficult position. Arizona expanded Medicaid under the health care law and it now covers nearly 30 percent of the state’s population. The Democratic Senatorial Campaign Committee has already launched ads targeting Flake on health care. Flake, whose state has experienced a 150 percent increase in premium costs from 2014 to 2017, is also sure to hear from concerned constituents on the issue.

Sen. Dean Heller, R-Nev.

Heller’s seat is another one Democrats are eying as a possible pickup opportunity next year. Medicaid covers roughly 20 percent of Nevada’s population and the provisions in the House bill targeting the entitlement program have been a key point of criticism for Heller. The DSCC is also targeting him in their attack ads on health care.

On the opposite side of the Republican political spectrum is a group of lawmakers hoping to shift the legislation further to the right. Several of them were highly involved in the House effort and urged the Freedom Caucus to push for a bill that repealed more provisions of the health care law than originally proposed by leadership.

As the debate in the Senate continues, bringing conservative and moderate lawmakers together will be the most difficult task for Republican leadership. And while there are ongoing attempts to find common ground, with unpredictable lawmakers such as Sens. Ted Cruz and Rand Paul in this camp, it is difficult to set expectations.

Sen. Ted Cruz, R-Texas

It is no surprise that Cruz, who once was a key player in shutting down the government for 16 days over funding for Obama’s health care law, is playing a key role in the current debate to unravel it. Now, as the fate of the legislation rests with the Senate, Cruz wields considerable influence within the conference. While he is up for re-election in 2018, don’t expect Cruz to calm his rhetoric on the topic.

Sen. Mike Lee, R-Utah

Lee, who like Cruz also involved himself directly in the House effort, has blasted that bill for codifying too much of the law. His state has the lowest Medicaid population in the country, but Utah has experienced substantial increases in premium costs since 2014.

Sen. Rand Paul, R-Ky.

Paul has introduced his own legislation to repeal the health care law that was modeled after the 2015 bill vetoed by Obama. While some of his fellow conservatives have expressed support for the bill, its fate was largely doomed in the Senate upon introduction. Still, Paul has found ways to make his criticisms of the repeal process known. He infamously scoured the House side of the Capitol with a copy machine looking for an alleged secret copy of the chamber’s bill that members were rumored to be viewing behind closed doors.

Sen. Patrick J. Toomey, R-Pa.

Toomey may be the chamber’s best hope for a compromise bill that could actually garner 51 votes. As proof of this, McConnell tapped Toomey to work with Portman on a solution to the Medicaid quandary. While he has sought to put a stricter cap on the amount of funding allocated to the states for the entitlement program, Toomey’s own state expanded the program under the 2010 health care law, which now covers nearly 20 percent of Pennsylvania’s population.

The effort to overhaul the U.S. health care system remains a Republican-only endeavor and it is expected to remain as such. While there have been some discussions across the aisle, Democrats’ refusal to negotiate with Republicans on any legislation that would repeal the health care law makes any serious attempt at bipartisan work a likely pipe dream at best.

While some of their states have experienced upward of a 50 percent increase in premium costs, as the Trump administration dangles crucial subsidy payments in front of insurers, expect Senate Democrats to hang much of the blame for the law’s problems on the current White House.

Sen. Joe Manchin III, D-W.Va.

Manchin was one of the first lawmakers to reach across the aisle. West Virginia was one of the states hit hardest by the ongoing opioid epidemic. Its Medicaid program, which the state expanded under the health care law, serves as a crucial lifeline for those recovering from addiction. Nearly 30 percent of the state’s population is enrolled in the entitlement program.

Sen. Claire McCaskill, D-Mo.

McCaskill is one of the GOP’s top targets for 2018. While her state has fared well under the health care law — premiums have only risen by around 15 to 20 percent — Trump handily won Missouri. For her part, McCaskill has introduced her own legislation to help those consumers with no insurer participation in the individual market in 2018.

Sen. Jon Tester, D-Mont.

Similar to Alaska, Montana is a state with significant rural health care issues. That remains top of mind for Tester, who is up for re-election in 2018. He has held a number of town halls this year on health care issues, Roughly 20 percent of the population in his state is covered by the Medicaid program.