Lawmakers avoided a government shutdown in the days before Christmas, but some of their most difficult fights remain ahead of them in the new year.

Congress passed a short-term spending measure that will fund the government through Jan. 19, by which time lawmakers will try to strike a spending deal for the rest of the fiscal year.

At the heart of the debate is the same fight that has plagued the annual appropriations process since 2011, when the Budget Control Act implemented caps on domestic and defense spending that have been periodically raised – almost always at the same pace. Republicans hoped again this year to increase the Pentagon's budget while demanding offsets for any increases in domestic spending. And while they would be able to pass such spending legislation themselves in the House, the Senate's rules that place a 60-vote threshold on most legislation means 52 Republicans will need at least eight Democrats on board.

But the must-pass nature of the spending bill means both sides will use it as leverage for other controversial issues.

Immigration

Democrats have planned to use that leverage to force the passage of legislation protecting young immigrants from deportation, after President Donald Trump issued an executive order rescinding the Obama-era Deferred Action Against Childhood Arrival program.

Protecting the group of young immigrants, who arrived in the U.S. before the age of 16, from deportation is one that enjoys broad popular support and the backing of many Republicans. Sen. Jeff Flake, R-Ariz., has led the negotiations among a bipartisan group of senators and the Trump administration to find a permanent legislative replacement for DACA, securing on Wednesday assurances from Senate Majority Leader Mitch McConnell, R-Ky., that he would permit a free-standing vote if they are able to strike a deal. Meanwhile, several dozen House Republicans have called on House Speaker Paul Ryan, R-Wis. , to take up the issue with urgency.

Although he canceled the program, Trump has said he favors a permanent legislative replacement to return the protected status. However, he has also suggested that he would only accept such a provision if it came tied to border security enhancements, as well as reducing legal immigration. In that vein, he has expressed support for a proposal from Sen. Tom Cotton, R-Ariz., and Sen. David Perdue, R-Ga., that would do away with the immigrant visa program known as the green card lottery, shift legal immigration toward a merit-based system and greatly curtail the ability for immigrants to follow other immigrant members of their families.

Debt Ceiling

Lawmakers struck a deal to suspend the debt ceiling last September through early December, but the Treasury has been able to use a process called extraordinary measures, borrowing money from other accounts to meet its obligations and to stretch its borrowing ability through sometime in January.

While the Constitution imposes the authority to raise the government's spending limit on the Congress, the prospect of increasing the Treasury's borrowing limit has only in recent years become controversial.

Deficit hawks oppose raising the debt ceiling as part of a broader effort to curtail spending, but failing to do so risks that the U.S. credit rating could be downgraded if the Treasury were unable to pay its bills. They know many of their colleagues will go to great lengths to avoid the debt crisis and will likely try to extract other spending concessions in exchange for allowing the hike.

Entitlement Reform

Fresh off achieving one of his dearest policy goals – the passage of a tax code overhaul – Ryan has shifted his sights to another item: entitlement reform. Even before the tax bill made it through the final hurdles, Ryan said cutting spending to Medicare, Medicaid and other welfare programs would be a primary focus of the House in 2018.

Unlike tax cuts, however, Ryan may find himself with less like-minded company. For many lawmakers on both sides of the aisle, Medicaid, Medicare and Social Security are sacrosanct, especially the latter two programs, which benefit the elderly.

Trump has gone back and forth on whether he would support cuts to entitlement programs; after taking an absolutist position during the 2016 campaign. He has suggested occasionally since then that he would be open to overhauling entitlements.

And while Democrats are certain to oppose Ryan's plan, he could also see opposition from his colleagues in the Senate. On Thursday, McConnell – mindful that his already tenuous advantage in the majority is about to shrink to a just two-vote 51-49 edge when Alabama Democrat Doug Jones takes office – said he did not expect welfare reform to come up in the new year.

"We have to have Democratic involvement. So things like infrastructure … to do something in that area we're going to have to have Democratic participation," he said .

Infrastructure

The administration earlier this month signaled plans to roll out its infrastructure proposal in January, following Trump's campaign promise to introduce a $1 trillion infrastructure package in his first 100 days.

Trump touted the yet-to-be rolled out plan following the recent Amtrak train crash in Washington state, and last week he said he wants Democrats' help.

"At some point, and for the good of the country, I predict we will start working with the Democrats in a Bipartisan fashion," he tweeted on Dec. 22. "Infrastructure would be a perfect place to start. After having foolishly spent $7 trillion in the Middle East, it is time to start rebuilding our country!"