The announcement that Montana Democrat Max Baucus is retiring shakes up the fight for the Senate in 2014 more than a bit.

Baucus has been mentioned as a GOP target, but only a second-tier one, far behind Republican opportunities in two open seats — West Virginia and South Dakota — and against two Democratic incumbents, Mark Pryor of Arkansas and Mary L. Landrieu of Louisiana.

But an open seat in Montana catapults that contest into the top tier as a potential Republican takeover opportunity.

Democrats are likely to try to woo former Gov. Brian Schweitzer into the race, but it’s unclear if he’ll be interested.

Without likely candidates, the only way of handicapping the race is to fall back on the fundamentals. And that means that Montana moves from leaning toward the Democrats to a pure tossup.

Republicans need to net six seats to take back the Senate next year, and that’s an uphill fight. But with Montana now becoming an open-seat contest, Democratic strategists know that the cycle suddenly looks more dangerous for them.