WASHINGTON (Reuters) - The Democratic-led U.S. Senate and Republican House of Representatives return this week for a showdown over government spending, disaster relief and defense policy before President-elect Donald Trump ushers in a new era of single-party rule next month.

The main challenge for lawmakers over the next three weeks is to avert a pre-Christmas partial government shutdown by striking a bipartisan deal to fund federal agencies beyond Dec. 20, when a current stopgap spending measure is due to expire.

The debate will include a nearly $100 billion emergency disaster relief request from President Joe Biden for areas of the U.S. Southeast hit by hurricanes Helene and Milton, and other communities struck by natural disasters.

Congress also faces a Jan. 1 deadline for raising the federal government's debt ceiling, though lawmakers and aides say that extraordinary measures employed by the Treasury Department are likely to postpone the expected "X" date for default well into 2025.

Senate Democrats, who are entering their final days in the majority, and some Republicans hope to enact an omnibus package of annual spending bills that would fund the government through fiscal year 2025, which ends on Sept. 30, rather than a shorter-term continuing resolution, or "CR."

"I still hold out this hope that we could avoid a CR," Senator Susan Collins, top Republican on the Senate Appropriations Committee, said before lawmakers left town ahead of the U.S. Thanksgiving holiday.

But Trump's allies are pushing for a three-month stopgap that supporters say would allow their party's incoming political trifecta to dismantle current Democratic spending initiatives and policy priorities early in the new administration.

"We've made omnibus spending bills painful to vote for (keep eyes on the Senate) ... now we must kill the practice," Representative Chip Roy, a leading hardline conservative, said on social media last week.

The House isn't expected to begin serious work on government funding until the last of the session's three weeks, with Speaker Mike Johnson expressing support for a continuing resolution that would run into early next year.

That strategy also poses risks for Johnson's slim 220-213 Republican majority, which failed to pass its own partisan stop-gap measure in September and had to rely on mainly Democratic votes to narrowly avert a shutdown weeks before the Nov. 5 election.

FIRST 100 DAYS

This time, Republicans aim to display greater unity ahead of gaining full control over fiscal 2025 funding early next year.

But the stopgap approach will also drain time and energy away from Trump's ambitious first-100-days agenda of tax cuts, energy deregulation and border security.

House Republicans will have a similarly narrow majority next year and could see their margin of error shrink to a single seat for several months, with the departure of Matt Gaetz and two other Republicans who are set to join the Trump administration.

The 100-day agenda of Trump's first presidency ran aground in 2017 over a similar funding question, forcing him to withdraw his controversial plan to finance a wall along the U.S.-Mexico border to avoid a government shutdown that April.

But Republicans believe they can enact Trump's agenda this time.

"There's no daylight between their agenda and what they envision and what we envision for the House," said Johnson, who has been in close contact with Trump.

Trump's transition team did not respond to a request for comment.

The CR strategy could help Johnson avoid the drawn-out January fight over the speakership Republicans experienced two years ago, as far-right members of the House Freedom Caucus like Roy vehemently oppose an omnibus spending package.

Hardline Republicans have been angered by Johnson's willingness to work with Democratic Senate Majority Leader Chuck Schumer on spending in the past.

Top lawmakers have yet to say how they intend to handle a Biden request for emergency disaster relief.

The head of the Small Business Administration recently testified to Congress that the agency's disaster loan program for homeowners, renters, and businesses ran out of money in October, leaving more than 60,000 loan applicants waiting for assistance.

"People are desperate for answers, and help, and hope, and they are looking to Congress for action," said Senator Patty Murray, Democratic chair of the Senate Appropriations Committee, "Everyday we don't act, the costs grow."

Aides said a disaster relief package would likely be attached to a CR.

But the first objective for Congress this month is likely to be passage of the National Defense Authorization Act, or NDAA, annual legislation that sets policy for the Defense Department, according to congressional aides. Floor votes could come as early as next week, according to aides.