WASHINGTON (AP) -- Republicans will face a critical test of their unity when a spending bill that would avoid a partial government shutdown and keep federal agencies funded through September comes up for a vote.

Speaker Mike Johnson, R-La., is teeing up the bill for a vote as soon as Tuesday despite the lack of buy-in from Democrats, essentially daring them to oppose it and risk a shutdown that would begin Saturday if lawmakers fail to act.

Republicans will need overwhelming support from their members in both chambers -- and some help from Senate Democrats -- to get the bill to President Donald Trump's desk. It's one of the biggest legislative tests so far of the Republican president's second term.

"The CR will pass," Johnson told reporters Monday, using Washington shorthand to describe the continuing resolution. "No one wants to shut the government down. We are governing, doing the responsible thing as Republicans. It's going to be up to Chuck Schumer and the Senate Democrats to do the right thing."

The strategy has the backing of Trump, who is calling on Republicans to "remain UNITED -- NO DISSENT -- Fight for another day when the timing is right."

House Republicans said the bill would trim $13 billion in non-defense spending from the levels in the 2024 budget year and increase defense spending by $6 billion, which are rather flat changes for both categories when compared with an overall topline of nearly $1.7 trillion in discretionary spending. The bill does not cover the majority of government spending, including Social Security and Medicare. Funding for those two programs is on auto pilot and not regularly reviewed by Congress.

Democrats are mostly worried about the discretion the bill gives the Trump administration on spending decisions. They are already alarmed by the administration's efforts to make major cuts through the Department of Government Efficiency, or DOGE, run by billionaire Trump adviser Elon Musk. And they say the spending bill would fuel the effort.

"This is not a clean CR. This bill is a blank check," said Rep. Rosa DeLauro of Connecticut, the top Democrat on the House Appropriations Committee. "It's a blank check for Elon Musk and President Trump."

Spending bills typically come with specific funding directives for key programs, but hundreds of those directives fall away under the legislation, according to a memo released by Senate Democrats. So the administration will have more leeway to reshape priorities.

"President Trump has endorsed this full-year CR because he understands what is in it for him: more power over federal spending to pick winners and losers and devastate Democratic states and priorities," the memo warned.

For example, the Democratic memo said the bill would allow the administration to steer money away from combating fentanyl and instead use it on mass deportation initiatives.

Normally, when it comes to keeping the government fully open for business, Republicans have had to work with Democrats to craft a bipartisan measure that both sides can support. That's because Republicans almost always lack the votes to pass spending bills on their own.

This time, Republican leaders are pushing for a vote despite Democratic opposition. Trump is showing an ability this term to hold Republicans in line. He met with several of the House chamber's most conservative members last week.

Now, House Republicans who routinely vote against spending bills said they would support this one. The House Freedom Caucus, which includes many of the House's most conservative members, issued a statement of support saying "contrary to Congress' longtime abuse of this legislative tool, this CR is a paradigm shift."

Rep. Thomas Massie, R-Ky., is still a holdout, though. He says he'll vote no.

"I guess deficits only matter when we're in the minority," said Massie, when asked why colleagues weren't listening to his concerns.

Trump went after Massie on social media, calling him a "GRANDSTANDER, who's too much trouble."

"HE SHOULD BE PRIMARIED, and I will lead the charge against him," Trump posted online.

Rep. Tom Cole, the Republican chairman of the House Appropriations Committee, acknowledged the continuing resolution was not the outcome he was seeking but said it was time to end the cycle of short-term extensions Congress has been passing to keep the government open. This will be the third for the current budget year.

"Congress does have other things to do," said Cole, of Oklahoma. "It's got a lot on its plate this year."

Meanwhile, House Democratic leaders have come out strongly against it. Less clear is how strongly they'll push members in competitive battleground districts to follow their lead.

"House Democrats will not be complicit in the Republican efforts to hurt the American people," House Democratic leader Hakeem Jeffries of New York said.

Senate Democrats generally seem to be emphasizing patience at this stage, waiting to see if Republicans can muscle the bill through the House before taking a stand.

"No comment," said top Democratic Sen. Chuck Schumer of New York as he rushed through the hallway outside the Senate chamber.

Still, several rank-and-file Democrats criticized the measure. Sen. Cory Booker of New Jersey said he was stunned that Republicans were "trying to jam through something that is their way or the highway."

If the bill does move to the Senate later this week, support from at least eight Democratic senators will likely be needed for it to advance to passage.

"It'll be up to the Democrats whether they want to deliver the votes and keep the government from shutting down," said Senate Majority Leader John Thune, R-S.D.

Democrats also introduced an alternative bill Monday night funding the government through April 11. The bill could serve as a Plan B if the GOP-led effort falters.

The spending bill could also have major ramifications for the District of Columbia's government. City officials voiced their concerns during a news conference outside the Capitol on Monday, and district residents later in the day flooded the hearing room and surrounding hallway where lawmakers were considering debate rules for the measure.

The bill would limit the district to last year's funding levels, though it's already spending at 2025 levels. Mayor Muriel Bowser, a Democrat, said the proposal would require the district to cut $1.1 billion in spending in the next six months since it has already passed a balanced budget and is midway through its fiscal year. That means, officials said, cuts to critical services such as education and public safety.

The mayor also emphasized that the district's 2025 budget focused on boosting three priorities: public safety, public education and economic growth.

"If the Congress goes through with this action, it will work against a priority that President Trump and I share, and that is to make Washington, D.C., the best, most beautiful city in the world," Bowser said.

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Associated Press writers Leah Askarinam, Gary Fields and Lisa Mascaro contributed to this report.