Meanwhile, the president and his GOP allies are asking for a significant funding boost for border security in order for Trump to sign the government spending bill. Senate Republicans are advocating for $5 billion, while the outgoing House GOP majority has pushed for as much as $25 billion — figures that Democrats oppose absent a broader deal on immigration.

If the two sides can’t come to terms, part of the government would shut down. Already, each party is heaping blame on the other, and the Senate is in session for only three weeks before the deadline.

“Is [Senate Minority Leader Chuck] Schumer going to shut down the government over border security? Strikes me as a bad idea,” said Senate Majority Whip John Cornyn (R-Texas) when asked about Mueller and the border wall.

“The president, the House and the Senate are all controlled by Republicans. If they want a shutdown, they’ll have a shutdown,” shot back Sen. Patrick Leahy (D-Vt.).

If Congress ends up punting by agreeing to a short-term funding bill, it would mean a steep cut in domestic spending favored by Democrats — while denying Trump a significant boost in border funding. Senate Democrats have signaled they could support $1.6 billion in border security money, though Trump and Republicans want more.

Congress took some of the sting out of a potential shutdown earlier this year by agreeing to funding for three-quarters of the government through next fall. But Homeland Security and several other departments were left out, and their funding is set to expire in December.

The prospect of even a partial funding lapse gives each party significant leverage to fight for its policy goals on two of the most divisive issues facing Congress.

When Congress set up the December showdown over some departments’ funding, the idea was to postpone a fight over the border wall until after the midterms. But a new issue has emerged in the past week: Attorney General Jeff Sessions was forced out and Democrats say they are not confident that acting Attorney General Matt Whitaker will protect Mueller from the president.

Schumer said Tuesday that Democrats will “attempt to add legislation to the must-pass spending bill in the lame duck session that will prevent acting Attorney General Whitaker from interfering with the Mueller investigation.” And other Democrats insisted that a bipartisan bill to protect the special counsel, which was approved by the Senate Judiciary Committee in April, must be on the agenda in the lame duck.

“It needs to happen now. Every day that goes by the special counsel is at risk,” said Sen. Elizabeth Warren (D-Mass.). “The Republicans assured us that the special counsel would be fully protected. And now the only way that we can guarantee that is if we pass a bill by a huge majority in the House and the Senate.”

Warren and other Democrats said that the Mueller provision doesn’t necessarily have to be in the spending bill. They just want a vote on the proposal during the lame-duck session.

“What would be acceptable to me is McConnell bringing our bill to the floor,” said Sen. Mazie Hirono (D-Hawaii). “That would make me satisfied.”

McConnell has said the bill is unnecessary, a view shared by most Republicans. Even Judiciary Chairman Chuck Grassley (R-Iowa), who supports the measure, said that while he’d vote for it on the floor, he would not insist it come up in the lame duck.

With McConnell unlikely to take action on the Mueller protection plan on its own, Schumer is suggesting that Democrats use their leverage over spending to ensure it passes.

“I don’t blame them, politically they can make a lot of fuss over it. ... To be honest with you, there’s not any reason to worry about it. Any president who would dump [Mueller] would be in real trouble,” said outgoing Sen. Orrin Hatch (R-Utah).

As Democrats maneuver on Mueller, Republicans are eyeing a big border wall funding boost. Hatch said that means more than $5 billion that Senate GOP leaders had previously sought.

“I think everybody wants to do border wall funding in a bigger way,” said Sen. John Thune of South Dakota, the No. 3 GOP leader who is expected to be promoted to whip during Wednesday’s leadership elections.

House Democratic aides say the party will take a stand against a Trump-style, physical concrete wall. But they’re open to a border security deal like the one agreed to early this year, which included new fencing that Republicans described as wall money. Plus, Democrats would rather clinch a spending deal now rather than haggle with it in January as they organize their new House majority.

But Democratic leaders say they aren’t willing to go beyond the $1.6 billion that was agreed to earlier this year.

“We think that’s more than adequate for reasonable investment in border security,” said Senate Minority Whip Dick Durbin (D-Ill.). “At this point, [Trump] can’t spend it. He can’t spend it responsibly. We know it’s a lead in to the 2020 cycle.”

House Republicans have already crushed multiple Democratic attempts to protect Mueller’s investigation by adding language to spending bills taken up at the committee level. Since Trump took office, Democrats on the powerful House Appropriations Committee have twice tried to force language that would block the Department of Justice from any attempt to “obstruct, hinder, frustrate, impede, or prevent” Mueller’s investigation.

House Democrats don’t believe House Republicans will go along with the bill to protect the special counsel. But with the House changing hands and unpredictable leadership elections underway, lawmakers are prepared for just about anything to happen ahead of Dec. 7.

“I’m not sure what the House is going to do,” said Sen. Patty Murray of Washington, the No. 3 Democratic leader. “You’ve been around long enough to know the chaos.”

Sarah Ferris contributed to this report.