It's more likely than not the government will shut down by midnight Friday. That's because a chunk of Senate Democrats and Republicans might vote against a short-term spending bill the House of Representatives just passed to keep the government open for another month.

Senate Majority Leader Mitch McConnell (R-Ky.) needs 60 votes, but there are only 50 Republican senators right now, and he can't even count on all of them to vote yes. Meanwhile, a majority of Senate Democrats seem ready to vote against the spending bill, calculating Republicans will get the blame.

Everyone who's prepared to vote no has different reasons (and political motivations). Here are the four key factions in the Senate that could determine whether the government shuts down for the first time since 2013.

1. Democrats up for reelection in November in red states

Sen. Joe Manchin III (D-W.Va.). (Carolyn Kaster/AP)

McConnell is pinning much of his hopes on getting a month-long spending bill through the Senate on these senators. His thinking boils down to: Will these red-state senators really vote to shut down the government — which could hamper the military and cause a number of states to run out of money for children's health insurance — because it doesn't have protections for those in the country illegally?

Answer: We don't know. Normally reliable “yes” votes on spending bills, most of these senators have been conspicuously quiet about what they plan to do, an indicator of the politically delicate situation they are in.

GOP strategist Alex Conant points out that a government shutdown usually gives a boost to challengers (i.e., people who weren't in Congress when it failed to do its most basic function). These Democratic senators were already vulnerable to being kicked out by Republican challengers, because they're running for reelection in states that Trump won.

But at the same time, these senators are likely getting pressure from their own party leaders not to vote with Republicans. Protecting “dreamers” is politically popular, too.

Who's in this category:

  • Joe Manchin III (D-W.Va.): He signaled he might vote for a spending bill. “I want to keep the government open. I'm just going to work and work and work to keep the government open,” he said.
  • Heidi Heitkamp (D-N.D.)
  • Debbie Stabenow (D-Mich.)
  • Joe Donnelly (D-Ind.): He said Friday he would support a spending bill. "“It’s the most basic duty of Congress to keep our government running."
  • Claire McCaskill (D-Mo.)
  • Bill Nelson (D-Fla.)
  • Tammy Baldwin (D-Wis.): She voted against the spending bill in December, siding with dreamers.
  • Sherrod Brown (D-Ohio): He voted against the spending bill in December, siding with dreamers.
  • Robert P. Casey Jr. (D-Pa.): He voted against the spending bill in December, siding with dreamers.

2. Democrats who are considering presidential runs

Sens. Cory Booker (D-N.J.) and Elizabeth Warren (D-Mass.). (Pablo Martinez Monsivais/AP)

It's only a slight exaggeration to say that every other Senate Democrat is considering or rumored to be considering a presidential run to unseat Trump in 2020. And when presented with a choice, these Democrats have consistently chosen the most liberal option. (A number of them support Medicare-for-all, despite the chances of that becoming law in a Republican Congress being zero.)

So it's little surprise that these Democrats are hard no's on this spending bill. They voted against a similar short-term spending bill in December, too.

Who's in this category:

  • Cory Booker (D-N.J.): “If the great dealmaker can’t do a deal when you control the House and Senate, that’s a massive failure on his part and any government shutdown is a reflection on his leadership,” he told reporters Tuesday.
  • Kirsten Gillibrand (D-N.Y.)
  • Kamala D. Harris (D-Calif.)
  • Amy Klobuchar (D-Minn.)
  • Chris Murphy (D-Conn.)
  • Bernie Sanders (I-Vt.): “It's on Republicans to stop a shutdown,” he wrote in a Jan. 8 op-ed in The Washington Post.
  • Elizabeth Warren (D-Mass.)

3. Democrats who face primaries in 2018

Sen. Dianne Feinstein (D-Calif.). (Reuters)

The more time that passes, the more pressure the Democrats' base puts on lawmakers to try to force a deal on dreamers through a Republican Congress. Perhaps no senator felt this more acutely in December than Sen. Dianne Feinstein (D-Calif.), who is the only Senate Democrat running for reelection and facing a potentially competitive challenger on the left.

California is also the state with the largest population of dreamers. And as The Post's David Weigel and Ed O'Keefe reported, she rapidly switched her position in a spending debate in December, first saying she would vote to keep the government open, then switching her vote at the last minute.

Supporters for a dreamer bill rally on the Capitol in December. (Yuri Gripas/Reuters)

She seems equally torn about what to do this time. She warned her party Thursday that when the government shuts down, “people die, accidents happen.” But she told reporters she's undecided on how to vote this time — contradicting a statement her office put out earlier saying she definitely wouldn't support a spending bill that doesn't have protections for dreamers.

4. Republicans who buck party leadership

Sen. Lindsey O. Graham (R-S.C.). (Joshua Roberts/Reuters)

Republicans' intraparty drama isn't just in the House. There are a handful of Senate Republicans who have voted, or say they will vote, against short-term spending bills, all for various reasons.

Some don't like that this will be the fourth short-term spending bill Congress has had to pass since the new fiscal year started in October. Others, like retiring Sen. Jeff Flake (R-Ariz.), side with Democrats and think Congress should deal with dreamers now. “We’re not going to get any better, particularly on the [immigration] issue, by waiting three weeks,” Flake told the Daily Beast.

Who’s in this category:

  • Flake
  • Lindsey O. Graham (R-S.C.): “I’m tired of it. This is the fourth one we’ve done, and you’re killing the military,” he told reporters Wednesday.
  • Mike Lee (R-Utah), a regular “no” vote on short-term spending bills
  • Rand Paul (R-Ky.), another senator who routinely bucks party leadership
  • Mike Rounds (R-S.D.): “It's not because immigration isn't included. For me, it's a matter of defense,” he told reporters Thursday.