Sen. Al Franken (D-Minn.) resigned Thursday in the face of sexual harassment allegations, and Democrats still seem to have a shot at pulling a major upset in the Alabama special election in four days' time.

But the biggest development this week in the 2018 battle for control of the Senate may have come much more quietly, when former Tennessee governor Phil Bredesen (D) announced Thursday that he'll run for the open seat being vacated by retiring Sen. Bob Corker (R-Tenn.).

Bredesen was always the Democrats' best (and perhaps only) hope of winning a seat in conservative Tennessee, but at 74 and having shown little appetite for national politics, it was anybody's guess as to whether he'd give it a go. He released a video saying he will.

Bredesen exited seven years ago as an extremely popular two-term governor, having won reelection in 2006 by a margin of more than 2 to 1 and carrying all 95 counties in the state. A Mason-Dixon poll conducted during his final year in office showed his favorable rating at 74 percent overall, even as then-President Barack Obama was at 34 percent. Bredesen actually had a slightly higher favorable rating among Republicans (77 percent) than Democrats (75 percent).

Running for federal office, of course, has often proved to be a tougher slog for centrist Democratic governors in red states. Former Ohio governor Ted Strickland (D) got walloped in his bid to unseat Sen. Rob Portman (R-Ohio) last year. Former Mississippi governor Ronnie Musgrove (D) came up short in his bid for a Senate seat in 2008. And in recent years, both Evan Bayh (D-Ind.) and Bob Kerrey (D-Neb.) have run as former governors who became senators, retired, and then ran for Senate again, only to fall by double digits. Political polarization tends to weigh much more heavily when you're talking about a vote in the Senate.

Bredesen didn't carve much of a national profile during his tenure, though he did endorse Obama in 2008 and was later rumored as a possible Obama appointee. Yet that hasn't stopped Republicans — whose candidates include Rep. Marsha Blackburn and former congressman Stephen Lee Fincher — from quickly labeling him an out-of-touch liberal and Hillary Clinton supporter.

Bredesen may not be the favorite, but he clearly puts the seat in play in a way Democrats could never have counted on in Tennessee — especially if Corker had sought reelection. And that's big for a pretty simple reason: They need three pickups if they are to have any hope of winning the Senate majority next year.

At the start of the cycle, Democrats only had two obvious ones, in Arizona and Nevada, while the rest of the map featured ruby-red territory. Since then, Democrats have been given a fighting chance in Alabama, thanks to the allegations against Roy Moore, and now they have a fourth potential pickup in Tennessee. If Democrats nab three of those four seats and hold all their territory, they'll win the majority. If they win Tuesday in Alabama, Bredesen's candidacy means they don't need to necessarily run the table.

That said, the math remains tough. Apart from their pickup opportunities, Democrats need to hold 10 states with Democratic incumbents that went for President Trump, including some by overwhelming margins, such as West Virginia, Missouri, Indiana and North Dakota. Adding an open seat in Minnesota, where Trump lost by less than two points last year, certainly isn't helpful.

But Democrats undoubtedly have a strong environment at this early juncture — a Quinnipiac University poll this week showed that Americans favored a generic Democrat to a generic Republican for Senate 51 percent to 37 percent — and the question has always been whether the map and recruiting would cooperate. While they can't be considered anything close to favorites to win the chamber, things are certainly moving in the right direction.

And if they do it, they may one day recall the date of Franken's resignation much more fondly than you'd think.