Montana Sen. Max Baucus is going to be appointed ambassador to China. Baucus, completing his 35th year in the Senate, had announced he would not run for re-election in 2014.

Baucus has been Chairman or ranking minority member on the Senate Finance Committee since 2001, a tenure unmatched since the days of Russell Long of Louisiana, who held those positions from 1965 to 1987.

National Journal reports that the next Democrat in line, Jay Rockefeller of West Virginia, who is also not running for re-election next year, will not take the Finance chairmanship, leaving it to Ron Wyden of Oregon. That should be interesting, since Wyden, like Baucus, has not always been a lockstep follower of the Senate Democratic leadership or the Obama White House (he was the one who asked National Intelligence Director James Clapper whether the National Security Agency was surveilling the telephone records of millions of Americans). Wyden has been to the left of the administration on this issue, but on economic issues he has often favored more market-friendly approaches than other Democrats.

Who will get Baucus’s Montana seat? Montana’s Democratic Gov. Steve Bullock can appoint someone to fill the remainder of Baucus’s term once he is confirmed and resigns from the Senate. One possibility is Lt. Gov. John Walsh, who is running for the seat. But Walsh has opposition in the Democratic primary from his predecessor as lieutenant governor, John Bohlinger, and one or two lesser-known candidates.

One possibility is to appoint a “seat warmer,” someone who can serve competently for the rest of the term but will not run for the full term. This is the course taken by Democratic Delaware Gov. Jack Markell. It is also the course urged by former Rep. Pat Williams, R-Mont.

Bohlinger's campaign manager suggests that the 76-year-old Williams be appointed as someone who is knowledgeable and won't be distracted by campaigning for the seat. That's actually a pretty strong argument, and it is the course taken in recent years by three former and current governors -- Delaware Democrat Ruth Ann Minner, West Virginia Democrat Joe Manchin and New Jersey Republican Chris Christie -- when faced with the task of appointing a senator to fill a vacant seat. Markell picked Ted Kaufman, a longtime aide to Joe Biden, who proved a competent and intellectually serious senator; Manchin appointed Carte Goodwin, a member of a politically very well-connected West Virginia family; and Christie appointed his attorney general, Jeff Chiesa.

It’s interesting to note that Baucus first won his Senate seat by beating an appointee who did run for the full term. At this point in the 1978 electoral cycle, Baucus, serving his second term in the House, was running for the seat from which three-term incumbent Democrat Lee Metcalf was retiring. In January 1978, Metcalf died. Democratic Gov. Thomas Judge, no fan of the 35-year-old Baucus, appointed state Supreme Court Chief Justice Paul Hatfield to fill the seat, and Hatfield set out to seek the full term. He voted to ratify the Panama Canal Treaty, although the state’s other Democratic senator, John Melcher, voted against it. In the June 1978 primary, Baucus beat Hatfield 65 percent to 19 percent.

Lesson: Voters like to choose their senators, not have their governors choose one for them.