The make-or-break moment for House conservatives to defund Planned Parenthood is here.

Outraged by undercover videos on fetal tissue donation, conservatives are pushing to cut all federal funding for Planned Parenthood as part of legislation that would avert a government shutdown on Oct. 1.

Rep. Mick Mulvaney (R-S.C.), a founding member of the House Freedom Caucus, spearheaded a letter to House GOP leaders shortly before lawmakers' August recess that stated he and 17 other Republicans would not vote for any spending bill that continues funding for Planned Parenthood.

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But with Congress set to return to Washington this week, it remains to be seen whether Mulvaney and his allies can gather enough support to force the hand of leadership.

Rep. Diane Black Diane Lynn BlackBottom line Overnight Health Care: Anti-abortion Democrats take heat from party | More states sue Purdue over opioid epidemic | 1 in 4 in poll say high costs led them to skip medical care Lamar Alexander's exit marks end of an era in evolving Tennessee MORE (R-Tenn.), who has written legislation that would freeze Planned Parenthood funding for one year while Congress conducts an investigation into its fetal tissue program, says she doesn't support using the government funding bill as the only leverage in the fight. Another option, for instance, could be attaching a measure to defund the organization to a reconciliation bill to repeal ObamaCare that would only need 51 votes in the Senate.

Black said she has secured a “commitment” from House GOP leadership that her bill will get a floor vote in the coming weeks, according to her spokesman.

“I wholeheartedly agree with the spirit and intent of my friend, Congressman Mulvaney’s letter,” Black said in a statement provided to The Hill. “The sad truth, however, is that President Obama is likely to veto any measure that combats funding of his political allies at Planned Parenthood.”

“I believe we must consider how to most effectively wage this battle while acknowledging our current political realities, positioning our movement for long-term success, and avoiding a government shutdown — which would not defund Planned Parenthood and would only embolden our radically pro-abortion president,” she added.

Mulvaney spokeswoman Stephanie Faile declined to specify the new total number of signatures on the letter, but said a “final count” would likely be released this week.

The letter would likely need to attract far more signatures to pose any real danger to passage of the stopgap funding bill that is expected on the House floor this month. At least one other Republican, Rep. Dave Brat (Va.), has added his signature to the letter over the August recess.

Still, 19 pledges falls short of the maximum number of defections GOP leaders can afford without needing help from Democrats to pass a spending bill. Assuming all 246 House Republicans are present to vote, GOP leaders can lose up to 28 of their members and still pass legislation on their own.

Ahead of the last government shutdown in 2013, 80 House Republicans had signed onto a letter pushing GOP leaders to defund ObamaCare.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellGraham: GOP will confirm Trump's Supreme Court nominee before the election Trump puts Supreme Court fight at center of Ohio rally The Memo: Dems face balancing act on SCOTUS fight MORE (R-Ky.) has been trying to lower expectations about trying to defund Planned Parenthood, saying last week that Republicans would have to wait for Obama’s successor in the White House.

“The president’s made it very clear he’s not going to sign any bill that includes defunding of Planned Parenthood, so that’s another issue that awaits a new president, hopefully with a different point of view about Planned Parenthood,” McConnell said on the Kentucky station WYMT’s “Issues and Answers.”

That argument is unlikely to sit well with conservatives eager for a confrontation.

“Establishment Rs say, "Wait till we have a GOP pres." If that happens, they'll say, "Wait till we have a filibuster-proof majority."#excuses,” Freedom Caucus member Rep. Justin Amash Justin AmashOn The Trail: How Nancy Pelosi could improbably become president History is on Edward Snowden's side: Now it's time to give him a full pardon Trump says he's considering Snowden pardon MORE (R-Mich.) wrote on Twitter.

And four GOP presidential contenders in the Senate — Ted Cruz Rafael (Ted) Edward CruzThe Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy Murkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight MORE (Texas), Marco Rubio Marco Antonio RubioMurky TikTok deal raises questions about China's role Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Florida senators pushing to keep Daylight Savings Time during pandemic MORE (Fla.), Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (Ky.) and Lindsey Graham Lindsey Olin GrahamGraham: GOP will confirm Trump's Supreme Court nominee before the election Fox's Napolitano: Supreme Court confirmation hearings will be 'World War III of political battles' Grassley, Ernst pledge to 'evaluate' Trump's Supreme Court nominee MORE (S.C.) — have said Congress should defund Planned Parenthood, likely making life harder for McConnell this month. Graham has said he doesn’t support forcing a shutdown over the issue.

Cruz, for his part, plans to send a letter to McConnell urging him not to schedule a vote on legislation that continues to fund Planned Parenthood.

“In light of recent and horrific revelations that Planned Parenthood is trafficking in fetal tissue and body parts from abortions, we urge you not to schedule or facilitate the consideration of any legislation that authorizes or appropriates federal dollars for Planned Parenthood," a draft of Cruz’s letter reads.

Aside from the funding fight, both the House and Senate will soon be debating resolutions to disapprove of the Iran nuclear deal, making it unlikely either chamber will have immediate floor time to address Planned Parenthood.

But the next steps are likely to be a hot topic on Wednesday morning, when House Republicans gather for their first conference meeting since the August recess.

- This story was updated at 9:52 a.m.