Welcome to the Senate, John Walsh.

Walsh (D-Mont.), who was appointed to the Senate on Friday and sworn in this week, is already facing attacks back home, with the GOP group American Crossroads launching a $125,000 ad buy focused on a reprimand that Walsh, the state's former adjutant general, faced from the U.S. Army in 2010.

The attack is not a new one, but the GOP group is inserting it into the discussion as Walsh begins his duties as senator.

Walsh is considered a slight underdog in the race for a full term this November. Rep. Steve Daines is the favorite on the Republican side.

Update: Walsh is up with a response ad, decrying the GOP "smears" of his record: