Udall first won in 2008, actually running slightly ahead of Barack Obama in the spread over Bob Schaffer. This year, Udall is on his own.
Presidential parties tend to lose seats in their last midterm election, but there are exceptions. Bill Clinton won 5 House seats and lost no Senate seats in 1998, but the general rule is incumbent presidents lose seats. G.W. Bush lost 6 Senate seats in 2006 and Ronald Reagan lost 8 in 1986.
Senate Democrats have 21 seats up, with several in states Mitt Romney won, like Arkansas, Alaska, Louisiana and Montana.
But, Colorado went for Obama by 5 points and the state party has been on a roll since the Ken Salazar senate win in 2004. Udall does have a couple of challenges. Races today are nationalized and polarized. The President has moved to the left on guns and immigration. Udall positions himself as an independent. He may need some “no” votes on preferred presidential proposals.
See Denver Post: Republicans searching for Colorado Sen. Mark Udall’s 2014 opponent