Sen. Al Franken (D-Minn.) on Capitol Hill in 2014. (Saul Loeb/AFP/Getty Images)

In the abstract, there’s nothing particularly weird about Sen. Al Franken (D-Minn.) announcing that he plans to resign his seat. It’s not something that most senators do, certainly, but it’s not unheard of. More than 300 senators have resigned at some point in U.S. history, several more than once. Often it’s because they’ve gotten new jobs — as in the case of Attorney General Jeff Sessions, who resigned earlier this year — and occasionally, as in Franken’s case, it’s because of scandal or rumors thereof. But it happens.

It’s just that if you had to look at the two senators from Minnesota and the histories of their seats and guess which of the two would end up having something weird happen, you’d have picked Franken.

It used to be that resignations from the Senate were much more common. Before the Civil War, there were an average of 2.7 resignations a year from the Senate. From 1866 onward, the average was less than one. You can see that drop-off if we look at resignations in five-year chunks since 1790.

The year with the most resignations was 1796, followed by 1974. During that latter year, an imminent change to the pension system prompted a number of senators to call it quits before Dec. 31.

Because Senate resignations were more common before the Civil War, older states tend to have had more instances of that happening. The state with the most resignations is Massachusetts. Minnesota has only had five, including Franken.

But again, the weirdness exists a level deeper. Let’s compare the two seats in Minnesota, which fall into Senate Classes 1 and 2. (There are three Senate classes as established by the Constitution; one is up for reelection in any given federal election year.)

The seat held by Sen. Amy Klobuchar (D) has changed hands 16 times since the state was founded. Franken’s has changed hands 25 times. Among those 25 changes there were five deaths and all five of the state’s resignations. Besides Franken, those resignations were:

  • William Windom, who resigned from the Senate to join the Cabinet, served less than a year and then came back to the Senate.
  • Hubert Humphrey, who resigned to become Lyndon B. Johnson’s vice president before returning to the Senate in the other seat, serving until he died. (Had he not left to serve as vice president, in other words, Franken’s seat would probably have six deaths in office.)
  • Walter Mondale, who took over for Humphrey and then resigned to serve as vice president to Jimmy Carter.
  • Wendell Anderson, who took over for Mondale.

Perhaps you’re thinking, Oh, so Anderson wasn’t as interesting as the other guys. Incorrect.

Anderson was governor of Minnesota when Mondale resigned and decided that a good senator to replace Mondale would be Wendell Anderson. So he resigned as governor, and his lieutenant governor, Rudy Perpich, appointed him to finish out Mondale’s term.

This was an unpopular move. The result was what came to be known as the “Minnesota Massacre,” in which Anderson’s party — the Democratic-Farmer-Labor Party, the ticket on which Minnesota Democrats run — got obliterated in the 1978 elections. Republicans won both Senate seats in the state that year — the Class 1 seat that was up for election and the Class 2 seat for which there was an election to finish out Mondale’s term (as the process works in Minnesota).

The weirdness around the seat continued. The next Democrat to hold it, Paul Wellstone, was killed in a plane crash shortly before he was up for reelection for a third term. Mondale, who’d once held the seat, was put forward to replace Wellstone on the ballot, but he was defeated by Republican Norm Coleman by about 50,000 votes.

It was Coleman whom Franken beat to win election to the Senate. But that race, you may recall, was even closer, with Franken ultimately given the victory by only about 300 votes. The election results weren’t finalized until April 2009, after recounts and legal fights.

It’s not known whom Minnesota Gov. Mark Dayton (D) — who once served in the Senate, holding the unexciting Klobuchar seat — will appoint to replace Franken. Given the history of the seat, whoever it is can probably expect a bumpy ride.