But none of that reduces the political calculations behind the move. Mr. Baucus’s early retirement leaves a Senate opening in a state that Mitt Romney carried by nearly 14 percentage points. By getting Mr. Baucus to leave early, Democrats increase the chances of holding a seat that could be decisive in which party controls the Senate in the last two years of the Obama Presidency.

Montana’s Democratic Governor Steve Bullock will be able to nominate a successor to serve through the next election, and the expectation is he’ll nominate his Lieutenant Governor John Walsh. Mr. Walsh is already running for the Baucus seat, and Mr. Bullock has endorsed him.

If Mr. Walsh is appointed early he might be able to head off a primary and he surely will gain more statewide name recognition. He’ll also be able to tap into the K Street business donors who want to hedge their bets that Democrats will retain Senate control. The political motivation behind all this is reinforced by the leaks to the media that Vice President Joe Biden and Obama political operative Jim Messina were central to negotiating Mr. Baucus’s nomination for Beijing.