ST. PAUL, Minn. (AP) — After a year that saw budget battles, a legal fight between branches of government and two lawmakers swept out of office over sexual misconduct allegations of last year, 2018 will start with another big event: Lt. Gov. Tina Smith heading to Washington to take a place in the U.S. Senate.

Smith’s move has triggered questions and bickering over who will succeed her at the state Capitol. It’s a critical question that has major ramifications for control of the Senate and could define the legislative session.

Here’s a look at what’s at stake:

HOW DID WE GET HERE?

The constitutional questions stem from Sen. Al Franken’s resignation, expected to be made official on Tuesday. Democratic Gov. Mark Dayton appointed Smith, his second-in-command, to take Franken’s place. She’ll be sworn in Wednesday.

The state’s constitution makes clear that the Senate’s current president — Republican Sen. Michelle Fischbach — immediately ascends to the open lieutenant governor seat. But Fischbach wants to keep her Senate seat too as Republicans cling to a narrow majority. That makes things ripe for a legal challenge; Democratic Senate Minority Leader Tom Bakk has promised a lawsuit, saying Fischbach should have to leave the Senate.

Republican Senate Majority Leader Paul Gazelka pushed for a special session to elevate a Democratic senator to Dayton’s side instead, but Bakk and Dayton didn’t bite.

“His interest is to take the majority of the Senate,” Gazelka said of Bakk. “It leaves me no other option but to fight.”

DOES HISTORY HOLD THE ANSWER?

History is split on how this might go.

Republicans point to an 1898 Minnesota Supreme Court case as evidence that Fischbach can hold both jobs. That old case upheld as constitutional a vote the then-lieutenant governor cast as a member of the Senate. At least three lieutenant governors since have kept their seats in the Senate even after ascending to the job to fill a vacancy, according to the state’s historical records.

But in other cases — most recently, in 1943 and 1976 — new lieutenant governors resigned from the Senate before taking their post. And Democrats say that a 1972 constitutional amendment that separated the lieutenant governor’s role as the formal leader of the state Senate, combined with a constitutional provision that bans legislators from holding another elected office, makes it clear Fischbach must resign her Senate seat.

WHEN WILL THIS BE SORTED OUT?

Both sides are digging in, so any hope for an easy resolution may rest on a special election for a Senate seat 120 miles from Fischbach’s central Minnesota district.

The Feb. 12 special election in Cottage Grove is to replace Democratic Sen. Dan Schoen, who resigned last month after sexual harassment allegations. The district has been in Democratic hands for more than a decade, but Republicans have made inroads in the area and recruited a longtime former House member to run.

If Democrats keep the seat, they’ll be in striking distance of reclaiming the majority if Fischbach is forced to resign. But their hopes for a majority would then depend on winning a special election for Fischbach’s seat — something Republicans scoff at. She won the conservative district by more than 37 percentage points in 2016. And Fischbach told KSTP-TV she’ll run for her seat in another special election if she’s forced out of office.

A GOP pickup of the Schoen seat would pretty much squash Democratic hopes of retaking control of the Chamber. Even with Fischbach out of office, the GOP would maintain a 34-32 majority.

Gazelka said he will fight any legal challenges that seek to force Fischbach out of her Senate seat.

WHAT DOES IT MEAN FOR 2018?

Lawmakers return to the Capitol Feb. 20 and won’t have forgotten a months-long legal battle with Dayton over his veto of the House and Senate operating budgets. The machinations surrounding Fischbach’s future didn’t help relationships between the two parties, either.

But the prospect of a deadlocked state Senate if Fischbach is forced out and Democrats win Schoen’s seat presents a logistical nightmare. Tied at 33-33 until a special election for Fischbach’s seat could be called, the Senate could grind to a halt.

Gazelka worries it could stymie important work to shore up financing of the state’s health care programs. He also hopes to tweak the state’s tax laws to help Minnesota residents who may be hit by Congress’s new tax bill, which caps deductions for income and property taxes to a total of $10,000 annually.

“If we’re at 33-33, we may get nowhere,” he said.