Funding for the U.S. government expires at 11:59 p.m. EDT this Friday, April 28. If Congress and the Trump administration don’t reach a deal, the government will go into shutdown mode, which means hundreds of thousands of federal employees will be put on furlough, national parks will be closed, government websites won’t be updated, and so on. This weekend has political significance as well: Saturday, April 29 will be President Trump’s 100th day in office. A government shutdown starting on that day would be fitting for a Trump administration that has struggled to accomplish many of its key initiatives, even though historically the start of a presidency is often one of its most productive times.

But will there be a shutdown?

Republican leaders in the House and Senate have hinted that there is an easy way to avoid a shutdown. In December, a bill authorizing funding through April 28 was passed by large margins in both the House (326-96) and Senate (63-36) and signed by President Obama, and congressional leaders would like to take the general funding formulas from that provision and extend them through September. That way, with the deadline pushed back, Congress and the Trump administration would have a few months to reconcile Trump’s controversial budget proposal with what Democrats and Republicans on Capitol Hill want.

But at least three groups in Washington could derail this approach. And in one sense, getting this bill through Congress is even harder than passing health care reform. Government funding bills can be filibustered, so this legislation needs not only 216 votes (a simple majority) in the House, but also 60 votes (a three-fifths majority) in the Senate, meaning at least eight Democrats must back it.

Here are the three groups to watch:

The Trump administration

The Trump administration, aware that this is a must-pass bill, is trying to attach some of the president’s priorities onto it — in effect forcing Congress to adopt them. The administration wants to include more than $30 billion in new defense and border security spending. That would include more than $1 billion to start building the wall on the U.S.-Mexico border, a core promise of Trump’s campaign. Trump’s team also wants more than $18 billion in cuts to domestic spending, in part to balance out those defense increases. And the administration has floated the idea of including provisions in this bill that would strip some federal funding for so-called sanctuary cities, which refuse to aid federal officials in enforcing national immigration law.

“We want wall funding. We want [immigration] agents. Those are our priorities,” Trump budget director Mick Mulvaney told the Associated Press. “We know there are a lot of people on the Hill, especially in the Democratic Party, who don’t like the wall, but they lost the election. And the president should, I think, at least have the opportunity to fund one of his highest priorities in the first funding bill under his administration.”

But most Democratic senators are strongly opposed to the wall and to punishing sanctuary cities. It is hard to see which eight Democratic senators would vote for these ideas. Even some Republicans in the Senate don’t fully support the wall, and others are wary of including funding for the wall in this particular bill, which would complicate its passage.

Ultimately, it’s not clear how much leverage the Trump administration has. If a Republican House and Senate adopted a bill that would avert a government shutdown, would Trump really veto it simply because it does not include some of his pet projects?

The House Freedom Caucus

When the government funding bill was adopted last December, the 96 votes against it came from a bipartisan group: 63 Democrats, some of whom thought it did not include enough spending, plus 33 Republicans. And if you followed the health care debate, many of the 33 Republicans who voted “no” will be familiar to you, including Michigan’s Justin Amash, Texas’ Louie Gohmert, Idaho’s Raúl Labrador, Kentucky’s Thomas Massie and North Carolina’s Walter Jones. The first three people on that list are known to be members of the House Freedom Caucus, while Jones and Massie are not members but often vote with the Freedom Caucus and against the priorities of House Republican leaders. All five of these House members were among the most vocal Republicans speaking out against the health care bill that Speaker Paul Ryan and President Trump supported.

So far, members of the Freedom Caucus have downplayed talk of a shutdown. But they have not clearly said that they are likely to vote for this bill either.

There are 237 Republicans in the House, and, because that chamber has five vacancies, it would take 216 votes to pass a bill right now. So if more than 21 House Republicans refuse to support the funding bill, it will either fail or need Democratic votes to pass. The Freedom Caucus, which does not publicly release its membership rolls, includes about 30 people, which is enough to tank the bill if they vote en masse against it.

The Democrats

The Democrats already have some influence here, since any funding bill will require at least eight of their votes to pass the Senate. But the House process will be important too. If it becomes clear that Republicans need Democratic votes in the House as well as in the Senate, Democrats could end up with a lot of leverage.

Democrats are now floating their own request: that Congress fully fund the cost-sharing subsidies that reduce out-of-pocket expenses for some people using the Affordable Care Act’s health insurance exchanges. Republicans in the House actually filed a lawsuit in 2014 to stop these subsidies, which are paid directly by the federal government to insurance companies. House Republicans said it was illegal for the Obama administration to make those payments without an explicit appropriation of money for them from Congress. A federal judge ruled in favor of the GOP position, although that ruling has been put on hold. The Trump administration could decide to stop the subsidies, which would anger health insurers and could cause them to stop participating in Obamacare. (The administration has not yet announced a formal policy.)

So Democrats want specific funding in this bill for those subsidies, which would placate insurers and presumably bring an end to the lawsuit. It’s hard to imagine congressional Republicans and Trump adopting any provision that supports Obamacare, but if Republicans need a large number of Democratic votes to get this funding bill through Congress, some Democratic ideas may get a hearing.

So will there be a shutdown? Right now, key figures in both parties, like Senate Majority Leader Mitch McConnell and Senate Minority Leader Chuck Schumer, are downplaying the possibility. And Congress has a lot of options it could use to avoid the worst outcomes, such as passing short (like a few days or weeks) extensions of government funding under existing law. That said, there was a 16-day shutdown in 2013, even as the country was led by a fairly traditional president. Now, House Republicans are deeply divided; the Senate still includes figures like Ted Cruz, who was instrumental in pushing for the shutdown four years ago; Trump is president and more unpredictable than Obama; and Mulvaney, one of the founding members of the Freedom Caucus, is a senior White House official. Anything could happen.