Congress has less than a month to prevent a government shutdown and conservative demands to defund Planned Parenthood are the biggest hurdle standing in the way.

The question plaguing all of Washington is just how big and lasting that obstacle will be.

The last thing Republican leaders want is another shutdown quagmire. But a group of conservatives — led by GOP presidential candidate and Texas Sen. Ted Cruz — has vowed to oppose any government spending bill that includes Planned Parenthood funds. That includes a stop-gap spending bill, known as a continuing resolution (CR), that needs to pass before Sept. 30.

Cruz stoked shutdown fears in August when he wrote that “funding the federal government does not require funding Planned Parenthood” in a USA Today op-ed. On Friday he began circulating a letter among Senate Republicans that will ask Senate Majority Leader Mitch McConnell (R-Ky.) not to schedule a vote on a spending bill that funds 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,” Cruz’s letter reads, according to a draft obtained by the Washington Post.

Cruz’s track record of dramatic standoffs on the Senate floor, paired with his presidential campaign built on taking on the Washington establishment, could turn into a recreation of the 2013 showdown over defunding Obamacare. Cruz led the fight that closed the federal government for more than two weeks over funding for President Obama’s signature law.

President Obama tells lawmakers a shutdown would be an "unforced error." (Reuters)

Yet, the ranks of lawmakers willing to risk the standoff were small when Congress left town in July for the August recess. But there’s mounting concern that more Republicans could join the defund bandwagon in reaction to anger from voters back home.

A July showdown over Planned Parenthood funding between GOP presidential candidate Sen. Rand Paul (R-Ky.) and McConnell was quickly diffused when McConnell offered Paul a separate vote to defund the group. That procedural vote failed to get enough votes to overcome a filibuster.

But a similarly happy ending to the burgeoning conflict may not be so easy when Congress returns this month.

McConnell doesn’t see much point in trying again. In an interview this week with WYMT in eastern Kentucky he reminded viewers that even if a bill to defund the organization passes the Senate, it would still be met with a presidential veto.

“We just don’t have the votes to get the outcome that we’d like,” McConnell said. “The president has made it very clear he is 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 and other Republican leaders are mulling options that would allow members to officially register their support for defunding the group without directly tying such a vote to government spending. The list of options of ways to tackle the issue and satisfy conservatives’ demands for action includes: official committee investigations, stand-alone legislation to fund separate health programs for women and a promise to include the issue in budget reconciliation later this fall.

So far, none has emerged a winner but it’s growing increasingly clear that investigations alone won’t be enough to satisfy the most ardent defunding backers.

If conservatives succeed in shutting down the government, or even spark fears of a shutdown possibility, GOP leaders will have to turn to Democrats for votes, handing Senate Minority Leader Harry Reid (D-Nev.) and House Minority Leader Nancy Pelosi (D-Calif.) huge leverage. Their first demand will almost certainly be to end the deep-cutting spending caps Republicans worked all year to maintain.

It is a familiar conflict for Republican leaders who have been forced to navigate intra-party brawls on an increasingly regular basis.

In July House Speaker John Boehner (R-Ohio) directed three committees to investigate Planned Parenthood, a move that was seen as an attempt to appease conservatives and channel their anger away from spending votes. The House Judiciary Committee has scheduled its first hearing in the investigation process for Sept. 9, days after members return to Washington

Judiciary has also asked the Department of Justice to launch an independent probe into the group’s practices.

But Heritage Action, the powerful political arm of the conservative Heritage Foundation, is pressing members not to take the bait.

“No pro-life member should be asked by party leaders to vote to continue funding Planned Parenthood,” the group said in a statement last week. “The Republican-controlled Congress must use the power of the purse to fully defund Planned Parenthood in September.”

That external pressure paired with a wave of anti-establishment fervor inspired in part by the continued success of Donald Trump on the campaign trail could give conservatives more confidence in a showdown with party leaders.

On July 18, House Republicans signed a letter vowing to oppose any spending bill that includes funding for Planned Parenthood. Several signers, including Reps. Jim Jordan (R-Ohio) and Mick Mulvaney (R-S.C.) who led the letter effort, say they don’t care if the vote to end federal funds for the group comes as a standalone measure, as part of a budget package or even as part of reconciliation, but they won’t vote for any spending bill that funds the organization.

“The specific vehicle matters less than the final outcome: We must redirect funds from Planned Parenthood to the federally qualified health clinics that provide more, comprehensive services to women and girls,” Jordan said.

Mulvaney said he doesn’t trust leaders to follow through on promises for future action. He says the only way to win his support for any spending bill is to kill the funds outright.

“A CR funding the government — and not funding Planned Parenthood– is real,” he said. “The promise of some future parliamentary procedure is not. We have seen that act before and somehow those promises never pan out.”

But Mulvaney and Jordan will have to recruit more than the original 18 letter signers if they plan to derail a CR on their own. Even if zero Democrats voted for the CR, Republican leaders could still stand to lose around 25 of their own members and still pass the funding bill.

One option that could eventually allow Congress to vote on the issue would be to add it to a budget reconciliation bill later this year. But such a move would not come soon enough to win back Jordan and Mulvaney or their backers.

Reconciliation bills are considered under special rules that require only a simple majority to pass. It would provide Republicans a chance on which to vote for, and likely pass, a bill to slash federal funds from Planned Parenthood.

But as McConnell pointed out, that bill would then be sent to Obama, who would surely still veto the entire package.