Sen. John Walsh (D-Mont.) (J. Scott Applewhite/AP)

This post has been updated with the news that Walsh has dropped his reelection bid.

It's a scenario that is extremely unusual, not only in Montana but nationally: a sitting senator who has won his party's nomination withdrawing from contention before the general election. That's what Sen. John Walsh (D-Mont.) did Thursday. It prompts two questions: How will he be replaced on the ballot, and by whom?

Walsh was appointed by Montana Gov. Steve Bullock (D) to fill the remaining term of former senator Max Baucus (D), who stepped down to become U.S. Ambassador to China. On June 3, Walsh won the party's nomination for a full term. It was going to be an uphill fight no matter what happened; he always trailed Steve Daines, the Republican nominee, in polling. And then the floor collapsed, with Walsh's admission that parts of his dissertation had been plagiarized. This week, his campaign had been canceling scheduled campaign stops and events; on Wednesday, he "took a personal day," according to campaign staff. And then he dropped his campaign on Thursday.

What happens next?

The first stop is at the Montana Secretary of State's office: Linda McCulloch, who holds that position, will first authorize the state Democratic Party to replace Walsh.

At that point, the party's Rule 15 kicks in. That rule articulates the replacement process: a special nominating convention held by the party at which a new nominee is selected. The party would send out written notices of the time and date of the special convention -- or, if time is running short, simply call the party delegates who are allowed to vote for a replacement candidate.

Those delegates represent a broad swath of party leadership from across the state: voting members of the executive committee, the chair and vice chair of the central committees for each of Montana's 56 counties, the state committeeman and committeewoman from each county, presidents of charter organizations, and so on. In total, the party estimates, it's about 175 people -- which is actually about 0.2 percent of all of the voters in the Democratic primary. That number is an estimate. Many counties might not have assigned committeewomen, for example, and not every county has a central committee.

The convention will likely be held in Helena, with the goal of making it "as open as possible," according to party spokesman Bryan Watt. The first priority will be voting delegates, of course, but Watt would also like to accommodate press and "as many people as possible," should such a convention become necessary.

Once the convention begins, the process gets a bit murkier. (Again: Uncharted territory.) Delegates will be able to nominate candidates from the floor, and those candidates will have a chance to speak. Then others could speak for or against the nominees. Finally, voting will begin. If none of the candidates got more than 50 percent of the vote in the first round, the lowest vote-getter drops from the ballot, and voting continues until there's a nominee.

When that happens, the party certifies the choice and sends it to the secretary of state -- and McCulloch starts the process of printing new ballots.

The window for all of this happening was short, however. If Walsh hadn't dropped out by Monday, he would have been the Democratic candidate. And if the party can't hold its convention and select a new nominee by Aug. 20 -- it's actually not clear what will happen.

Update 5:08 p.m.: State Democratic Party Chairman Jim Larson says in a statement: "The Montana Democratic Party looks forward to an open and transparent opportunity for Democrats to come together to decide our new nominee. As we move forward in this race, we must remember this election is not about one person; it’s about Montana."

Who's in line to replace him?

The names that have been floating around:

  • John Bohlinger. Bohlinger was lieutenant governor of the state under Gov. Brian Schweitzer -- and was a Republican at the time. When Baucus left his seat, Bohlinger ran in the primary to replace him, losing to Walsh in June by more than 40 points.
  • Nancy Keenan. Keenan is the former superintendent of public instruction in the state and former president of NARAL Pro-Choice America, an abortion access advocacy group. She spoke at both the 2008 and 2012 Democratic presidential conventions.
  • David Wanzenried. Wanzenreid currently sits in the state Senate, representing the area around Missoula.

There's another name, too, that appears above: Schweitzer. The former governor had reportedly been considering a run for the presidency in 2016, until an article in the National Journal included some colorful quotes from Schweitzer about his views on identifying homosexual people. A Senate bid, which he had previously turned down, might now look more appealing for Schweitzer.

Update 6:44 p.m.: Schweitzer won't run.

That's the thing about the party's process. It values relationships in a way that the primary process doesn't (at least, not directly). In some ways, it's a throwback to the days when state legislators picked senators -- until national reforms were instituted after a senator was caught bribing his way into the office. That senator was William Clark, of Montana.

Has this ever happened before?

According to Kate Scott of the U.S. Senate Historical Office, there's been one other time (that she was able to find) in which a sitting senator earned his party's nomination but resigned before the general election. And it was recently, too.

In 2002, Sen. Robert Torricelli of New Jersey won his party's nomination for reelection, but dropped out of the race after it was revealed that he'd received illegal campaign contributions from a businessman with ties to North Korea. The state Supreme Court had to weigh in, allowing the Democratic Party to replace Torricelli's name with former Sen. Frank Lautenberg, who went on to win the election. (Sidebar: Lautenberg and Torricelli loathed one another, making the swap all the more amazing.)

The path to Washington is much more complicated than that for any potential replacement for Walsh. Torricelli resigned in part because it was clear that he would lose reelection in a strongly Democratic state. Walsh stepping down might improve the chance his party holds the Senate seat, but it still will be something of a long shot.