WASHINGTON (AP) — Confronting their party’s most powerful leaders, moderate Democrats are tapping the brakes on President Joe Biden’s multitrillion-dollar domestic program and insisting on their own priorities first in a smaller infrastructure deal. The interparty showdown is headed for a test vote Monday evening in the House.

The band of moderates has threatened to oppose a $3.5 trillion budget blueprint unless the House first approves a $1 trillion package of road, power grid, broadband and other infrastructure projects that’s already passed the Senate. They could conceivably sink the fiscal blueprint in the narrowly divided House.

With most of Biden’s domestic agenda at stake, it’s unimaginable that House Speaker Nancy Pelosi, D-Calif., would let her own party’s centrists deal him an embarrassing defeat. That’s especially true with the president already under criticism over his handling of the U.S. withdrawal from Afghanistan and with Democrats’ prospects uncertain in the 2022 elections for control of Congress.

Pelosi implored lawmakers Monday afternoon to come together, saying there is no time to waste. “We must not squander our Congressional Democratic Majorities and jeopardize the once-in-a-generation opportunity to create historic change to meet the needs of working families,” she said in a letter to colleagues before the session.

Members of the House returned to Washington on Monday in what Democratic leaders hope will be just a two-day interruption of lawmakers' August recess to make gains on Biden's priorities. Leaders want quick approval of the budget resolution, which will set the stage this fall for setting further legislation directing $3.5 trillion at safety net, environment and other programs over the next decade. Lawmakers were scheduled to huddle privately before the vote for a caucus meeting.

That huge measure comprises the heart of Biden's vision for helping families and combating climate change and is progressives' top priority, all of it largely financed with tax increases on the rich and big business.

But the moderates want Congress to quickly send the smaller, bipartisan infrastructure measure to Biden so he can sign it before the political winds shift. That would nail down a victory they could point to in their reelection campaigns next year.

“The House can't afford to wait months or do anything to risk passing" the infrastructure bill, Rep. Josh Gottheimer, D-N.J., said late last week. He's a leader of the nine moderate mavericks who each released statements reaffirming a desire that the infrastructure vote come first, and others may join them.

Pelosi, backed by the White House, is leading her party in a tightly-scripted strategy that aims to keep moderate and progressive lawmakers on board for what would be a landmark federal investment and the cornerstone of Biden's domestic policy agenda. In the narrowly divided Congress, to pass any legislation over solid GOP opposition, Democrats can lose no more than three votes in the House.

The Monday evening procedural vote on the infrastructure measures, as well as a voting rights bill, another top Democratic goal, will test the strategy ahead.

Some solution averting a Biden setback in the House seems likely. Leaders were in talks with lawmakers signaling they want both infrastructure bills passed by Oct. 1, an ambitious schedule. Pelosi, top House Democrat since 2003, has a long history of doing what it takes to line up the votes she needs on important issues.

White House press secretary Jen Psaki on Monday underscored Biden’s support for Pelosi’s plans. Psaki deemed it a “healthy debate” within the party and said that it was “a high-class problem to have” as Democrats debate the particulars of the legislation.

So far, neither the moderates nor the powerful forces confronting them were showing signs of budging. The party's progressive members are vowing to withhold their votes on the infrastructure bill unless the larger measure focused on expanding child care, Medicare and providing for paid family leave is passed.

Biden met virtually with Pelosi and other Democratic leaders and committee chairs late last week. In a show of solidarity, the White House and Pelosi issued similar statements afterward underscoring their determination to approve both measures soon and pointedly ignoring moderates' demand to do infrastructure first.

Unless the moderates decide to oppose the procedural measure, Democrats controlling the chamber 220-212 should be able to push it through.

Among moderates, Rep. Henry Cuellar, D-Texas, said in an interview, “No progressive is going to cram something down my throat.” Rep. Jared Golden, D-Maine, said failure to pass the infrastructure bill quickly “leaves the nation's economy and crumbling infrastructure hostage to political gamesmanship.”

On the other side, progressive leader Rep. Katie Porter, D-Calif., said in an interview that Democrats “not actively supporting” Biden's priorities “are not moderates,” suggesting they're conservative.

The group Justice Democrats, which recruits progressive candidates including challengers to congressional incumbents, released a fundraising appeal saying Gottheimer was being supported by "the worst of the political establishment.” It did not identify them.

Lawmakers from both parties began Monday making their pitches prior to this week's votes.

Democratic lawmakers framed the votes as a chance to build on the $1.9 trillion COVID-relief bill that Congress passed earlier this year that provided $1,400 stimulus checks to most Americans, boosted unemployment insurance payments and expanded the child tax credit, among other things.

“Long before the pandemic, decades of federal under-investment in climate, education, housing, childcare, health care and other sectors have made it harder for American families to make ends meet and for American businesses to complete globally,” said Rep. John Yarmuth, the Democratic chairman of the House Budget Committee. “We must close these deficits now.”

Republicans said the $3.5 trillion effort that Democrats are seeking to advance fails to address “the crisis that American families are facing” and would lead to higher inflation and deficits.

“The inflation crisis, the border crisis, the energy crisis, the Afghanistan crisis — this budget only makes it worse,” said Rep. Jason Smith of Missouri, top Republican on the House Budget Committee.

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AP Congressional Correspondent Lisa Mascaro and Jonathan Lemire contributed to this report.