U.S. Sen. Richard Blumenthal (D-CT) is spearheading an effort from 42 senators — enough to stage a filibuster — to remove what they argue is anti-LGBT language from the annual defense bill. (Photo by Alex Wong/Getty Images)

Senate Democrats are insisting that negotiators drafting a final defense policy bill leave out a provision that would allow federal contractors to discriminate against workers on the basis of sexual or gender orientation, an issue that could become a major political fight in the post-election “lame-duck” session.

“Our government should have no part in funding discrimination,” reads a letter penned by Sen. Richard Blumenthal (D-Conn.) and signed by 42 Democrats and independents – enough to filibuster the defense bill if they hold the line. It is being delivered to the chairmen of the House and Senate Armed Services Committee on Tuesday.

The letter comes just two weeks before lawmakers are expected back in Washington for a lame-duck session, when Congress will be under a serious time crunch to pass the annual legislation that authorizes military programs. Negotiators came close to striking a deal on the measure before Congress left town in September ahead of the election, but came up short on a handful of provisions, including what do to about the sage grouse and how to handle LGBT protections.

Democrats are not explicitly threatening to filibuster the bill if it contains he LGBT provision, but the number of senators signing on to the letter makes it clear that is an option.

“This letter sends a message about how strongly and widely-held our belief is that this amendment would seriously jeopardize existing important workplace non-discrimination provisions,” said Blumenthal, a member of the Armed Services Committee and the conference committee negotiating the final defense bill.

The dispute started in April, when the House Armed Services Committee adopted an amendment from Rep. Steve Russell (R-Okla.) exempting religious organizations with government contracts from federal civil rights law and the Americans with Disabilities Act. The amendment would effectively override President Obama’s 2014 executive order prohibiting federal contractors from discriminating against LGBT workers. Democrats also worry it could allow for discrimination against women, based on their reproductive health choices.

The provision was never part of the Senate bill. But the House GOP appears to be digging in over the amendment, just the latest stage in a half-year effort to make sure it becomes law.

Earlier this year, House GOP leaders muzzled an effort to excise the Russell amendment from that chamber’s version of the defense bill. House Democrats later led a successful effort to include language banning federal contractors from discriminating against LGBT individuals in a bill to fund energy and water programs, but the victory was short-lived because that bill failed to pass the House.

The back-and-forth played out as Republicans and the Obama administration faced off over the president’s executive order mandating public schools have bathrooms to accommodate transgender students, a struggle that strikes similar political themes as the fight over the defense bill’s LGBT provision. Prominent GOP members suggested earlier this year that the House would try to undo Obama’s executive order on transgender bathrooms as well.

Obama’s veto warning against the House’s defense bill, which names the Russell amendment among its objections, has not dissuaded Republican negotiators from insisting the provision be included in the final defense bill. Many religious conservatives in the party see efforts to remove the amendment as an assault on religious freedom.

Blumenthal’s letter is the latest effort to label the Russell amendment as a poison pill that Democrats will not swallow.

Opponents of the provision have been directing their efforts to get rid of it toward Senate Armed Services Committee Chairman John McCain (R-Ariz.). This week, the Human Rights Campaign’s government affairs director David Stacy again called on McCain to “stand with the majority of fair-minded Arizonans — and Americans all across the country — and drop this discriminatory provision.”

Last month, 89 House Democrats signed onto a letter similar to Blumenthal’s, appealing to House Armed Services Committee leaders to remove the language. But the Senate’s letter carries more weight because of the threat of a filibuster.

Blumenthal would not say whether Senate Democrats are prepared to block the defense bill over the LGBT language, insisting their goal is “to assure that it’s eliminated without reaching the floor.”

If it isn’t, Blumenthal said Democrats senators would “consider other options.”