It’s Christmas again in Congress.

Members in both chambers return to Capitol Hill on Monday from a ten-day recess with four weeks left to put together a massive fiscal 2018 spending bill. And the package, which Congress must pass by March 23 to avoid another government shutdown, may be the last major legislative vehicle to advance this year.

That turns up the pressure on lawmakers, who must now begin the annual tradition of lobbying House and Senate leadership to include in the package their “pet projects” that, for one reason or another, are unlikely to advance unless attached to a vehicle — congressional speak for a major bill that is large enough to carry other pieces of legislation.

Omnibus bills in recent years have become as synonymous with passing unrelated policy measures as they have with the annual appropriations process, so much so that the spending legislation is generally referred to as a “Christmas tree” bill.

Watch: What Policy Measures Could Get Added to the Spending Bill?

As much as Congress relies on the massive package to act as a buffer after failing to advance the annual 12 appropriations bills, leadership also uses it to pass unpopular legislation. Other times the vehicle is packed with the pet projects that otherwise wouldn’t make it to the floor.

In the weeks leading up to the unveiling of the omnibus, reporters pepper lawmakers with questions about which bills will “hitch a ride” on the spending package.

This year will be no different. But since members know this could be their last big legislative vehicle — that is, until the next omnibus comes around — everyone is feeling the heat. 

Below are five policy measures that could be part of the debate over the upcoming package.

Stabilizing the insurance markets

Lamar Alexander of Tennessee, chairman of the Senate Health, Education, Labor and Pensions Committee, and Sen. Patty Murray of Washington, the panel’s top Democrat, have for months pushed their legislation intended to help stabilize the insurance markets created by the 2010 health care law.

The bill would, among other things, fund the so-called cost-sharing subsidies that reduce out-of-pocket costs for some lower income individuals and give states more flexibility to implement the law.

The measure has strong bipartisan support — Senate Minority Leader Charles E. Schumer once pledged that all Democrats in the chamber would vote for it.

But in the aftermath of actions by the Trump administration to tinker with the law, Democrats are seeking new additions to the legislation, including increased funding for the law’s premium tax credits.

Such a provision would likely face backlash from Republicans, especially conservatives in the House, raising doubts among some that the measure could eventually reach President Donald Trump’s desk.

Funding reinsurance

Sens. Susan Collins of Maine and Bill Nelson of Florida are pushing a separate bipartisan health care-related measure that would provide $4.5 billion over two years for federal reinsurance, a program that helps insurers recoup losses for covering the sickest individuals.

The bill has broad support in both parties and even has subtle backing from House Speaker Paul D. Ryan. He has not expressed similar support for the Alexander-Murray bill.

Senate Majority Leader Mitch McConnell may feel added pressure to attach the Collins-Nelson bill to the omnibus after promising the Maine Republican a vote on the measure in exchange for her support of the GOP tax bill signed into law last year.

Immigration stalemate

The Senate’s freewheeling and open debate this month on how to address the pending expiration of the Deferred Action for Childhood Arrivals program became, as expected, a partisan battle resulting in no measurable progress. Passage in the House of any DACA-related measure seems equally dicey.

Now, advocates are scrambling to figure out how to pass something before the program — which covers undocumented immigrants who come to the country as children — ends on March 5, the deadline imposed by President Donald Trump when he terminated it.

It is unlikely any sort of DACA patch will be included in the spending bill. McConnell, Ryan and other GOP leaders have sought to separate the immigration debate from the spending measure. And some Republicans believe they have ample time to act as Trump’s decision to end the program is debated in the federal courts.

Watch: How The Senate Immigration Debate Stalled

Election tampering

With the 2018 elections looming, Democrats are already requesting that additional funding for the Federal Bureau of Investigation be added to the upcoming spending bill to help protect against foreign influence in the midterms.

Schumer, House Minority Leader Nancy Pelosi of California and other Democratic leaders want an additional $300 million in funding for the FBI included in the omnibus.

Watch: Intelligence Officials Aware of Russian Activity Aimed at 2018 Elections

And a handful of others have introduced bills in the past few months intended to further bolster the security of the U.S. election system and prevent Russian influence in the elections.

Senators like Democrat Chris Van Hollen of Maryland and Republican Marco Rubio of Florida might make a push to get their respective measures added to the spending bill.

Gun control

After a gunman killed 17 students and faculty at a high school in Florida this month, lawmakers from both sides of the aisle are renewing calls for legislation to bolster background checks, ban some categories of guns and improve the federal reporting process to prevent some ineligible people from purchasing firearms.

It seems unlikely that a Republican-controlled Congress would advance any major gun control overhaul, but a few smaller measures could move this year after months of stalled action — and the spending bill could be a vehicle for those.

Among the candidates is legislation from Senate Majority Whip John Cornyn of Texas and a group of bipartisan senators that would improve state reporting to the National Instant Criminal Background Check System.

But the measure, which would seek only to ensure that states comply with existing federal law, faces backlash from conservatives who have due process concerns.

And House Republicans added a concealed carry provision to the bill before passing it out of the chamber, jeopardizing its chances. That addition would face steep resistance from Senate Democrats.

Watch: Cruz Talks Gun Legislation, Obamacare at CPAC