President Obama's sinking approval numbers are hurting the reelection chances of Sen. Mark UdallMark UdallLive coverage: Tillerson's hearing for State The rise and possible fall of the ‘Card’ in politics Gardner's chief of staff tapped for Senate GOP campaign director MORE (D-Colo.), according to a new survey from Democratic firm Public Policy Polling.

Udall leads former GOP Senate nominee Ken Buck by 46 percent to 42 percent, a closer result than in earlier polls of the race. His approval rating is in slightly negative territory, with 41 percent disapproving and 40 percent approving.

Obama's approval rating is down to 43 percent with 54 percent disapproving, down from a 48 percent to 50 percent spread when PPP polled the state in April. Obama won the state with 51 percent of the vote in 2012.

Many have expected Udall to have little trouble winning reelection, but this is the latest poll showing him in a tough fight in the swing state, despite a weak GOP primary field. Buck, a flawed candidate, lost to Sen. Michael Bennet (D-Colo.) in 2010 after making a series of gaffes on social issues but holds a big lead in the primary, according to the poll.

The automated poll of 928 registered voters was conducted from Dec. 3-4 and has a margin of error of plus or minus 3.2 percent.