The U.S. government could shut down tonight at midnight. Here's why.

Here we go again — the Senate is still negotiating aspects on a key spending bill, and the lack of action could lead to a shutdown at midnight Friday.

The debate centers around coal miners' benefits, Politico reported: The Senate Republican majority has already laid out their funding plan and say they don't plan to keep negotiating even on the coal miner provision.

But Democrats, particularly those from coal states, are pushing back by fighting the entire government funding measure — which could lead to a shutdown if the spending bill isn't passed by tonight's deadline.

A government shutdown is a funding gap created by Congress not appropriating necessary funds to run the U.S. government. During a shutdown, non-essential employees are furloughed and many agencies' services are disrupted.

Senate Majority Leader Mitch McConnell (R-Ky.) told reporters that Democrats need to take "yes for an answer," or else they'll be responsible for the shutdown:

The issue is serious enough for Sen. Joe Manchin (D-W.Va.) that he postponed a planned meeting with President-elect Donald Trump at Trump Tower on Friday. Manchin, who is reportedly in the mix for a Cabinet position for the incoming administration, will now meet with Trump on Monday, a spokesman for the senator said.

Manchin insists that Democrats will settle for nothing less than a year-long extension of health-care benefits for those miners. But McConnell told POLITICO they will have to settle for what they have now and continue the fight in 2017.

... Republicans said that Democrats were not getting enough blame for their brinkmanship over the mining matter.

"I'm still waiting for CNN to do that little time clock they always do every time Republicans are about to close down," said Nevada GOP Sen. Dean Heller. "12-15 hours we'll be out of here. And that means 1-2 o'clock in the morning."

One possible solution is that Democrats would agree to Republicans' demands as they're written, as long as they get an assurance from the GOP to reconsider the coal miner benefits in 2017.

The House of Representatives already passed the spending measure, and many of them have already left town for the holidays. Now it's up to the Senate to pass the measure by midnight.