ST. PAUL, Minn. — A high-ranking Minnesota lawmaker thrust into the lieutenant governor's office by U.S. Sen. Al Franken's resignation abruptly took the oath of office and resigned her state Senate seat on Friday, a swift reversal for the longtime Republican senator who had resisted the job and its duties.

Senate President Michelle Fischbach's ascent was automatic after Democratic Gov. Mark Dayton appointed then-Lt. Gov. Tina Smith to the U.S. Senate following Franken's resignation amid a sexual misconduct scandal. Guarding a one-seat majority in the state Senate, Fischbach and fellow Republicans insisted for months she could remain in the Senate even after being elevated to lieutenant governor.

Her decision to resign from the chamber curtails a second lawsuit from a constituent challenging Fischbach's dual roles. But it also triggers the sole state Senate election of the fall, a contest that will determine control of Senate. The rest of the state's 66 Senate districts aren't on the ballot until 2020.

Fischbach's departure leaves the Senate deadlocked until January, but unless Dayton calls them back into special session — and he has said he won't — they are done for the year. And while some state lieutenant governors cast tie-breaking votes, Minnesota's does not.

After being sworn in Friday on a family Bible, Fischbach said the recent end of the legislative session changed her calculus and freed her to leave the legislative body she had served in since 1996. Dayton plans to call a special election for the Senate seat coinciding with the Nov. 6 general election.

The move creates a rare mixed-party administration: Dayton is a proud liberal Democrat, while Fischbach is a staunch conservative.

"When we disagree, we'll disagree. I'm used to that," he quipped, a reference to the acrimonious end of the legislative session this week.

Fischbach had avoided taking the oath for nearly five months and fought a pair of legal challenges since she ascended to become Dayton's lieutenant governor on Jan. 3, as she clung to her Senate seat.

While she and fellow Senate Republicans argued there was plenty of historical precedent for a presiding officer of the Senate also serving as lieutenant governor, Democrats called it a blatant constitutional violation of the ban on holding two elected offices.

An initial lawsuit filed by Democratic constituent Destiny Dusosky was dismissed earlier this year, in large part because it was filed before the Legislature resumed its session — and thus before Fischbach had carried out both roles. Dusosky followed up with a second challenge in April. That case was awaiting a hearing scheduled for June 5.

The legal challenge had heavy implications, as Fischbach gave Republicans a one-seat edge in the Senate. Whether forced by the courts or voluntary, her exit triggers a high-stakes election that will determine control of the Senate. Her central Minnesota district is heavily Republican, and GOP leaders scoffed at the notion that Democrats could win it.

Minnesota Public Radio News reports Fischbach told colleagues in an email she would not run for the seat she has held since 1996. She had previously indicated she would run in a special election if she were forced from her seat.