Former Montana governor Brian Schweitzer is not planning on running for an open Senate seat. (Drea Cunningham/The Washington Post)

Former Montana governor Brian Schweitzer’s stunning decision not to seek an open Senate seat in the Last Best Place is an unexpected boost for Republicans’ chances of retaking the majority in 2014 — although it is by no means determinative for the GOP.

Schweitzer’s no-go decision is bad on two levels for Democrats. In Montana, it takes a seat considered a likely hold for the party and turns it into — at least at first glance — a likely pickup for Republicans. Nationally, Montana becomes the third open Democratic seat — West Virginia and South Dakota are the others — for which the party’s chances look slim, a major development given that Republicans need six seats to reclaim the majority.

Let’s start with the Montana implications of Schweitzer’s announcement.

Schweitzer’s candidacy was assumed in the political world following the surprise retirement announcement of Sen. Max Baucus (D) in the spring. The popular ex-governor remains voraciously ambitious in the political arena, and the Senate seemed like a decent stop on the way to what many people in and out of the state thought might be a run for president in 2016.

The field was effectively frozen as Schweitzer made up his mind. With him not running, look for Rep. Steve Daines (R) to come under heavy pressure to make the race. And while Democrats talk about state schools superintendent Denise Juneau and state auditor Monica Lindeen, neither woman has the proven electoral record (or even close to it) of Schweitzer.

It’s worth noting that Democrats have demonstrated their ability to win in Montana — even with a national wind blowing in their collective face. Sen. Jon Tester won a second term in November despite the fact that President Obama won just 42 percent of the vote in the state. But that was a race featuring a Democratic incumbent. Montana in 2014 will be an open seat.

Nationally, Montana becomes the third problematic open seat for the party. In West Virginia, Rep. Shelley Moore Capito (R) is a clear favorite, as Democrats have yet to persuade a serious candidate to run. In South Dakota, the two leading potential Democratic candidates took a pass while popular former governor Mike Rounds dodged a serious Republican primary challenge.

If you give Republicans those three open seats — they are favored at the moment, but the election remains 16 months away — they then need three more for the majority. Those pickups would almost certainly come from four seats, all of which are held by Democratic incumbents running for reelection — in Alaska, Arkansas, Louisiana and North Carolina.

That fact was the silver lining Democrats focused on in the wake of the Schweitzer decision Saturday. “Only three Democratic incumbents have lost reelection in the last decade,” noted Guy Cecil, executive director of the Democratic Senatorial Campaign Committee.

Of the quartet of targeted Democratic senators, Arkansas’s Mark Pryor looks to be the most vulnerable — particularly if freshman Rep. Tom Cotton (R) decides to run. While Pryor is a known commodity in the Natural State, the fact that Obama won just 37 percent of the vote there in 2012 is a massive hurdle for any Democrat.

Alaska and Louisiana are not much friendlier for Democrats; Obama took 41 percent in each of those states. But a contentious Republican primary seems to be shaping up in Alaska for the right to take on Sen. Mark Begich (D), and in Louisiana, Sen. Mary Landrieu (D) has proved she knows how to win close races, claiming reelection victories in 2002 and 2008 with 52 percent of the vote or less. In North Carolina, where Obama won in 2008 and took 48 percent in 2012, the landscape is more level for Democrats, although freshman Sen. Kay Hagan (D) is regarded by both parties as endangered.

To win back the majority, Republicans need to win Montana, South Dakota and West Virginia and beat three of the four Democratic incumbents mentioned above. They could also, theoretically, expand the playing field a bit wider, although recruiting failures for Iowa’s open seat lessened the party’s chances of a pickup there. And of course, they have to hold the seat of Senate Minority Leader Mitch McConnell (Ky.), the only Republican incumbent in any real political peril.

Make no mistake: Schweitzer’s decision not to run gives Senate Republicans more flexibility to get to 51 seats in November 2014. But the path to a GOP majority still goes through Democratic incumbents in Republican-leaning states.