Republican hopes for moving an ambitious tax package in a closely divided Senate may hinge on a number of incumbents on the ballot, including Luther Strange of Alabama, who faces a tough primary runoff on Sept. 26.

The vulnerability of Republican incumbents like Strange underscores the challenges facing Senate Majority Leader Mitch McConnell as he tries to hold together at least 50 votes in his 52-member conference to pass a partisan tax plan under a filibuster-proof reconciliation bill.

Senior Republicans praise Strange, appointed in February to the seat vacated by Jeff Sessions when he became attorney general, as a team player. They voice doubts about his primary rival Roy Moore, a former state Supreme Court chief justice, in potentially crucial floor showdowns on taxes and other issues.

Senate Majority Whip John Cornyn pointed to Moore’s record of being twice removed by a judicial ethics panel in Alabama for defying federal court orders: once in 2003 for refusing to remove a Ten Commandments monument from a state building and again last year for urging probate judges to deny marriage licenses to gay couples.

“Getting thrown off the Supreme Court of your state twice I don’t think is a credential that commends you for membership in the United States Senate,” said Cornyn, a former Texas Supreme Court justice.

Of the prospect for more GOP primary challengers like Moore during the 2018 midterm elections, Cornyn said, “We’ve seen that sort of thing happen before. And it’s not a pretty picture.”

Wrench in the works

Ross Baker, a political science professor at Rutgers University, said a tough political environment would ensure an uphill slog for a GOP tax package.

“I would put the probability of their completing a tax bill at about 30 percent. It is arguably the most complex public policy area, and touches the largest number of interest groups of any issue,” Baker said.

President Donald Trump plans to campaign for Strange in Alabama on Friday, and has recorded a robocall telling voters that Strange is “going to get the tax cuts for us.” On the other side, Moore has criticized McConnell as a creature of the “Washington swamp” and accused the top Republican of running a “slime machine” that fails to reach conservative goals.

The stalled push to uproot the 2010 health care law has angered Republican conservatives and energized primary challengers such as Moore, who are urging GOP voters to hold incumbents like Strange accountable for not delivering legislative victories.

Since Alabama is a GOP stronghold, the winner of the runoff will be the favorite in the special general election against Democrat Doug Jones on Dec. 12 and will serve the remainder of Sessions’ term. The Alabama Senate seat would be up again in 2020.

Steven L. Taylor, a political scientist at Troy University, said Moore or Strange would be likely to weigh home-state concerns first in deciding on components of a GOP tax package.

“The reality is that any tax vote is going to affect different constituencies in different ways. If you have a revenue-neutral bill, that by definition will make the vote difficult. For example, if there’s a reduction in the mortgage interest deduction to pay for a lower corporate rate, that would be difficult,” Taylor said.

Baker believes the GOP base will play a role in the tax debate.

“I would be very astonished if they came up with something heavily in favor of the top 1 percent,” he said about Republicans crafting a tax package. “The party base is really riled up by populists. I don’t think that would be acceptable.”

Beyond Alabama

The Alabama runoff could provide a preview of what lies ahead next year for the eight Senate Republicans up for re-election. Some Trump allies, including former White House strategist Steve Bannon, are considering whether to recruit or assist GOP candidates to the right of incumbents such as Bob Corker of Tennessee, Jeff Flake of Arizona, Dean Heller of Nevada and Roger Wicker of Mississippi next year.

Corker, who has questioned Trump’s competence for the presidency, has said he is undecided about running for another term.

Earlier this summer, Corker helped lead an effort to remove a proposal that would have repealed the 3.8 percent net investment tax for wealthy taxpayers from the Republican bill to repeal and replace the 2010 health care law. Now, the Budget Committee member is making the case for a revenue-neutral tax package, instead of pursuing one that would add to the deficit.

“I’m willing to give them some headroom here on the way things are scored and some of the things that we go through here. But, at the end, I want to make sure myself that it’s going to generate growth, we have broadened the base and, importantly to me, we are not going to do something that is going to increase our deficit,” Corker said Tuesday about a tax bill.

Heller, a Finance Committee member, has voiced support for exploring a bipartisan tax accord, but made clear he would work with party leaders if they opt to move a GOP-only tax plan.

“I’m going to guess that at the end of the day, it’s going to be a partisan exercise,” the Nevada Republican said.