A new survey conducted for a group backing Colorado Sen. Mark UdallMark UdallGardner's chief of staff tapped for Senate GOP campaign director The untold stories of the 2016 battle for the Senate Colorado GOP Senate race to unseat Dem incumbent is wide open MORE (D) shows him locked in a tight competition with Rep. Cory GardnerCory GardnerMcConnell huddles with GOP rep to discuss possible ND Senate opening GOP senator: Congress will ‘undo the damage’ from Obama Senate GOP to Obama: Stop issuing new rules MORE (R) for one of the state's Senate seats. 

The automated telephone survey, conducted by Democratic pollster Public Policy Polling for the League of Conservation Voters, gives Udall 47 percent support among registered voters to Gardner’s 43 percent support, results within the poll’s 4-point margin of error. Ten percent of respondents were undecided.

Other surveys, both Democratic and Republican, have shown a similarly tight race for the seat, which became more competitive for the GOP when Gardner jumped in earlier this year.

But the environmental group trumpeted the polling results as evidence that its $1 million ad campaign — criticizing Gardner for his ties to billionaire brothers Charles and David Koch and his support from the oil industry — launched in April is working.

The survey indicates that as voters have gotten to know Gardner, they’ve viewed him more negatively. Thirty-five percent view him favorably, while 42 percent view him unfavorably in this new survey. That's up from 23 percent with a positive opinion and 25 percent with a negative opinion of him in a March survey. A quarter of voters remain undecided.

However, while the new survey shows registered voters are much more evenly split over Udall’s job performance, it shows him dipping underwater since that March poll, when 41 percent approved and 40 percent disapproved of his job performance.

Now, 45 percent disapprove of his job performance, while 42 percent approve, and 13 percent aren’t sure.

The survey was conducted among 526 Colorado registered voters May 7-8 via landline phone.