Senate Democrats argue that the House-passed health care bill contains a flaw that would prevent Republicans from using a powerful legislative tool to move the measure over the minority party’s opposition. If the Senate parliamentarian agrees, that could prove a formidable obstacle to GOP plans to clear a major bill revising the 2010 health care law.

Democrats want the Senate parliamentarian to rule that the House bill ran afoul of the complex rules of so-called budget reconciliation, due to a provision that touches on the Indian Health Service. The House bill repeals a section of the 2010 law that created cost-sharing subsidies. Democrats say the provision that would be repealed falls into the jurisdiction of the Indian Affairs Committee, which was not specifically designated as a participant in the House’s detailed reconciliation plan in the fiscal 2017 budget resolution.

If the Democrats win support from Senate Parliamentarian Elizabeth MacDonough, she could find a “fatal” violation with the House bill, critics of the bill say. That may mean Republicans would lose the privileged status for the bill, which shields it from a filibuster requiring a 60-vote threshold. With control of only 52 seats, Republicans wouldn’t be able to overcome Democratic resistance if they lost the ability to use the reconciliation process.

A solution to a fatal error might be fixed in the House with a new vote, but the measure squeaked through the chamber on May 4 on a 217-213 vote. GOP leaders may struggle to repeat that success. On May 24, the Congressional Budget Office said the bill would increase the ranks of the uninsured by 23 million over a decade.

Oregon Sen. Ron Wyden said Tuesday that Republicans deliberately picked a tricky strategy for their health care bill in opting for reconciliation.

“Their choice was to do it this way, hardball, reconciliation, partisan only, ‘put a gun to everybody’s head,’” said Wyden, the ranking Democrat on the Finance Committee and a senior member of the Budget panel.

There’s been debate in Congress for decades about whether reconciliation is intended for enacting major health care legislation. The late Sen. Robert C. Byrd, D-W.Va., a jealous guardian of the chamber’s tradition of debate, in the 1980s spearheaded rules that limit reconciliation. In the 1990s, he dashed Democratic hopes for using reconciliation to move a Clinton administration package past a hostile GOP. Byrd also opposed this strategy in the early months of the Obama administration as well. “I am certain that putting health-care reform and climate change legislation on a freight train through Congress is an outrage that must be resisted,” Byrd wrote in a 2009 opinion piece in The Washington Post.

Democrats later in 2009 moved a health care bill through regular order, making a significant, but ultimately futile, effort to win over even a single Republican. The Senate in a 60-39 vote in December 2009 passed the bulk of the health policy provisions in the Obama administration’s signature law. The surprise election of Sen. Scott Brown, a Massachusetts Republican, then cost Democrats their 60th vote. After House and Senate Democrats agreed to changes to the bill, the party turned to reconciliation to clear those through a second smaller measure that became part of the package. Byrd was among the 56 Democrats to vote for that reconciliation measure.