Emily's List, the largest financial contributor to Sen. Kyrsten Sinema's campaign for office, is threatening to cut off its support of the Arizona Democrat if she keeps refusing to agree to change the Senate's filibuster rules in a push to pass the party's voting legislation.

"We want to make it clear, if Sen. Sinema cannot support a path forward for the passage of this legislation, we believe she undermines the foundations of our democracy, her own path to victory and also the mission of Emily's List, and we will be unable to endorse her moving forward," Laphonza Butler, the organization's president, said in a statement Tuesday, reports The Washington Post.

Emily's List, the nation's largest funding and endorsement source for female Democrat candidates who support abortion rights, believes that access to voting and confidence in election results is vital to its mission, Butler added.

"Sen. Sinema’s decision to reject the voices of allies, partners, and constituents who believe the importance of voting rights outweighs that of an arcane process means she will find herself standing alone in the next election," she said.

Emily's List has joined with "many others" to impress to Sinema that the voting rights legislation is important, but "those concerns have not been addressed," Butler added.

The powerful organization contributed $405,588 in Sinema's 2018 race for the Senate, eclipsing her next fundraising source by almost $200,000, according to Open Secrets.

Sinema, in response to the threat from Emily's List, said it is vital to keep the filibuster, and that the procedure has been used repeatedly in the past "to protect against wild swings in federal policy, including in the area of protecting women’s healthcare, reports The New York Times.

"Different people of good faith can have honest disagreements about policy and strategy," the Arizona Democrat said. "Such honest disagreements are normal, and I respect those who have reached different conclusions on how to achieve our shared goals of addressing voter suppression and election subversion, and making the Senate work better for everyday Americans."

Sinema, along with Sen. Joe Manchin, D-W.Va., have both said they will not vote to change the filibuster rules, which would be needed to overcome the Republicans' opposition to the voting bill.

She said she does support the voting legislation, but that she will not support a single party dismantling the filibuster, which would require votes from 60 senators to stop debate on most legislation, because she believes doing so will stop chances for chamber bipartisanship.

But even if Sinema backs off that opinion, Manchin said Tuesday he has not changed his mind on cutting the filibuster.

Republicans are united in opposing the legislation, which they say will give too much control of the nation's elections to the federal government.

Emily’s List has been facing pressure from its donors and from liberal activists to take a stand in the voting legislation showdown. The Times reports the group was Sinema's biggest donor in her last Senate race in 2018.

Some potential primary challenges for her 2024 race, including Rep. Ruben Gallego of Arizona, have been speaking out about the Emily's List donations, and two Senate powerhouses, Sen. Elizabeth Warren, D-Mass., and Sen. Bernie Sanders, I-Vt., have addressed the issue.

Warren declined to rule out backing a primary challenge to Sinema, but Sanders hinted he could support challengers against both Sinema and Manchin.

"Anybody who believes in American democracy has got to vote to enable us to go forward with 50 votes to suspend the filibuster," Sanders said Tuesday.