Once again, Congress is staring at the edge of the abyss.

Lawmakers return to session next week with just four days to fund the government and avert a shutdown. The deadline is April 28.

The dynamics are different this time, compared with the 2013 meltdown. There’s a Republican House and Senate. This is the first government funding go-round with President Trump occupying the White House. No one is quite sure how the Trump administration will handle the negotiations or what are their untouchable requests. But there’s not a lot of time to figure this out. Some Republicans fret that House GOP leaders burned way too much time trying to rescue their stunted health care bill.

A lapse in government funding would represent the second major legislative failure by Trump and the Republican Congress. A shutdown, following the failure to repeal and replace ObamaCare, could prove politically catastrophic for the exclusive, governing party in Washington.

But here are the keys. First, funding the government could, yet again, hinge on ObamaCare. Secondly, while Republicans run Washington, Democrats hold many of the cards in this poker game.

The House GOP’s stumble to repeal and replace ObamaCare before the recess didn’t appear to have a direct connection to the pending government funding battle. But now it may. Just days ago, Trump declared he would yank subsidies known as “cost-sharing reductions,” or CSR’s, from ObamaCare programs. The government directs the CSR payments to insurers who grant coverage to low-income people. A dried-up subsidy could force insurers to drop ObamaCare and spike premiums for the poor.

Trump views the ObamaCare subsidies as leverage to force Democrats to the table on health care. Democrats contend the president is holding the health care assistance “hostage” and imperiling those who aren’t well off. Trump has engaged with few Democrats since taking office on addressing ObamaCare or funding the government. Those who lose coverage (many of whom backed the president last fall) will know precisely who forced them to lose coverage should Trump successfully strip the subsidy. Still, there’s no better place to withdraw the subsidies than in the upcoming spending bill. One would think the president would drop the CSR’s in this spending bill if he’s serious about the new policy.

But that is a poison pill. Republicans may love the idea. However, it torpedoes any notion that Democrats might support the spending package.

“The spending bill cannot be done by one party alone,” opined Senate Majority Leader Mitch McConnell, R-Ky., before the recess. “These bills can’t pass without a reasonable number of Democratic supporters in the Senate.”

Therefore, is the president willing to stick to his guns on the ObamaCare subsidies or test the possibility of a government shutdown?

Blame the Democrats for this? Well, it’s hard to do that when Democrats don’t formally control any of the levers in Washington.

This is why it’s hard to make good on campaign promises. The rhetoric sure sounds lofty in the cornfields of Iowa and the snows of New Hampshire. But now?

Speaking of campaign promises, how’s funding for that wall going? It’s unclear if Trump will insist lawmakers attach money for the border wall to this upcoming spending package. But you can bet that Democrats will again bolt if that scenario comes to pass.

Wouldn’t the president latch money to construct the wall to this spending bill if he were serious about the project? But then again, Trump probably could get the wall and fail to keep the government open, too.

Plussing-up military dollars? Senate Armed Services Committee Chairman John McCain, R-Ariz., is clear the Pentagon needs a jump-start in funding. That’s something else on which Trump campaigned. But Congress operates under the Budget Control Act of 2011. That plan capped what’s called “discretionary” spending for years down that road and created “sequestration,” the budgetary phenomenon of arbitrarily limiting various spending pots regardless of need. Under the Budget Control Act, the “discretionary” spending ceiling (excluding entitlements like Medicare, Medicaid and Social Security) for this cycle is $1.07 trillion. Pouring in additional money for the military (or, for that matter, the wall) busts those sequestration limits. Keep in mind that many fiscal conservatives in the House and Senate want to spend less overall. That’s one of the reasons Republicans need to lean on Democrats for votes to keep the lights on in Washington.

One of the best ways to determine the musculature of a policy is to calculate how much money Washington devotes to a given initiative.

How about stripping sanctuary cities of federal dollars? A good place to execute that policy would be a rider in this spending bill. Democrats would interpret such an approach as another poison pill and balk at voting for such a measure.

With the deep uncertainty over whether Congress can address all of this in such a short timeframe, there’s already discussion of punting and adopting a stopgap measure of a week or two.

But are these policy promises idle threats or does the president insist on Congress including such provisions in the spending bill? Does Trump concede on a few subjects and let Democrats score some wins? Do they fail to work any of this out and spark a government shutdown?

Congress completed much of the work on the spending bills behind the scenes over the past few months. Back in December, Democrats only wanted to fund the government through late March. Republicans demanded late April and prevailed. Never mind that House Speaker Paul Ryan, R-Wis., promised to abandon the now common practice of bundling together spending bills rather than advancing them individually. That approach could come later this year. But Congress certainly didn’t do any of that ahead of this spending deadline.

So something has to give. And yet again, Congress stares into the abyss.

Capitol Attitude is a weekly column written by members of the Fox News Capitol Hill team. Their articles take you inside the halls of Congress, and cover the spectrum of policy issues being introduced, debated and voted on there.