(CNN) — House Speaker Paul Ryan warned the White House and Democrats Tuesday that Republicans won’t back down from a fight over government spending that left unresolved could result in a mid-December shutdown.

“This is the legislative branch, and the power of the purse rests within the legislative branch. And we fully expect that we are going to exercise that power,” Ryan said when pressed over whether he planned to attach so-called “policy riders” to a must-pass spending bill that Congress needs to approve before December 11.

The two-year budget deal that President Barack Obama signed on Monday sets the overall levels for government spending, but in order to avoid another government shutdown, the House and Senate need to approve a funding bill detailing levels for all federal agencies.

Earlier this year, Republicans and Democrats feuded over federal money for Planned Parenthood after a series of videos produced by an anti-abortion group suggested the group was selling fetal body parts for profit.

Planned Parenthood denied any illegal activity, but the GOP-led House set up a select committee to continue investigating the matter and many GOP members insist that the next spending measure bar any additional taxpayer funding for the group. Other conservatives want to push other policy provisions to push back at the Obama administration’s regulatory policies.

Ryan enters the first week on the new job aimed at fulfilling promises he made in order to win support for the speaker’s job. He’ll have his challenges to do just that, with the December deadline looming and a contentious fight over a long-term highway bill that must be passed before Thanksgiving.

Part of Ryan’s promise: To allow a more bottom-up approach and decentralize power from the speaker’s office.

In his first extended meeting with House Republicans, Ryan made clear that he would make some changes, according to lawmakers present. One of those changes: He would alter the makeup of the House Steering Committee before Thanksgiving — a key demand of conservatives who want more say in the decision-making process.

That panel, typically dominated by the speaker’s office, makes important decisions on assigning members to plum committee spots, but Ryan has privately said he would install a more diverse group of members to help dole out those assignments.

In one of the first decisions of the Steering Committee, Republicans will vote Wednesday to install either Reps. Kevin Brady of Texas or Pat Tiberi of Ohio as a successor to Ryan heading up the powerful tax-writing Ways and Means Committee. Ryan refused to say which man he’d like for the job, an effort to show his colleagues he is not dictating decisions from the speaker’s office.

Moreover, Ryan made clear that he would let the House work its will, signaling he would not stand in the way of a controversial amendment this week to revive the moribund Export-Import Bank, the nation’s chief export credit agency that is reviled by small-government conservatives. While Ryan opposes the bank, the business community strongly backs it — putting Ryan in the crosshairs in the first big fight of his speakership.

“It’s a test of leadership right off the bat,” said Rep. Mike Conaway, R-Texas.

Democrats are already warning of a confrontation over GOP efforts to condition continued federal spending on policy provisions. In an interview with the Huffington Post published on Monday, Sen. Chuck Schumer said there “may be” a shutdown, adding that members on what he called “the hard right” could “demand a shutdown.”

“It won’t be over the numbers. We won that fight. We won it big. It would be over the riders,” the New York Democrat said.

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