President Obama's sinking approval numbers are hurting the reelection chances of Sen. Mark UdallMark Emery UdallSenate GOP rejects Trump’s call to go big on gun legislation Democratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups MORE (D-Colo.), according to a new survey from Democratic firm Public Policy Polling.

Udall leads former GOP Senate nominee Ken BuckKenneth (Ken) Robert BuckHouse conservatives to air grievances in new 'Swamp' docu-series America has a broken political system our leaders need to fix Dem seeks to curb tax breaks for employee buyouts over sexual misconduct MORE 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.