A new poll has Rep. Ron Barber early lead over his likely - but hypothetical for now - Republican opponent Martha McSally.

Commissioned by the left-leaning House Majority PAC, the survey of 400 likely voters has Barber ahead with 45 percent of the vote, compared to 37 percent backing McSally.

The remaining 18 percent of those surveyed by the Democratic polling group Normington, Petts & Associates were undecided. The margin of error for the poll was 4.9 percent.

Matt Thornton, the communications director for the House Majority PAC, said the poll in the hotly-contested district show Barber has the edge despite the influx of attack ads financed by the Tucson Democrat's political enemies.

"As you can see, Barber has survived the Koch brothers-backed LIBRE attacks and is in a strong position," Thornton said.

The poll also hinted only a third of respondents gave McSally a favorable rating, compared to nearly half who thought Barber was doing a good job.

Unable to provide an exact breakdown of the party registration for the poll, Thornton said it was an accurate representation of the CD2 electorate.

McSally Campaign Manager Weston McKee said the poll had little value, noting the firm only released partial numbers and was hired by a group that has run television ads on local airwaves supporting Barber.

One of the the super PAC's goals is helping Democrats nationwide win seats in the House.

"It's no surprise that Ron Barber is relying on Nancy Pelosi and her liberal Washington allies to mislead voters in Arizona. Without releasing their methodology, these numbers are meaningless," McKee said. "How can we trust Ron Barber's personal hit group, which has already dropped $350,000 in attack-ads in a seat they are desperate to hold onto?."

A poll released earlier this year by the McSally teams gave the Republican candidate a slight lead ahead of Barber - 45 percent to 42 percent. However, the poll has a margin of error of nearly five percent.

The House Majority PAC has ordered a $1 million in air time to run ads in Tucson and the Phoenix markets this October.

Ashley Nash-Hahn, spokeswoman for Ron Barber for Congress, said the new ad shows that the McSally ad got it all wrong.

"As Eric Cantor learned this week, Republican pollsters often miss the mark," Nash-Hahn said. "Despite what McSally's recent poll said, our community overwhelmingly approves of Ron Barber's work on behalf of the people he represents, and that is why he holds a lead even after the Koch brothers' onslaught of negative ads."

Nash-Hahn said the poll also shows CD2 voters are playing close attention to the race five months before the general election.

"This poll shows that Southern Arizonans continue to reject McSally's empty slogans and her refusal to be honest about her positions on the issues. The message is loud and clear: Southern Arizonans want a leader who will give them straight answers," Nash-Hahn said.

Contact reporter Joe Ferguson at jferguson@azstarnet.com or 573-4346. Follow him on Twitter @JoeFerguson