Democrat Russ Feingold has a 6-point lead over incumbent Sen. Ron JohnsonRonald (Ron) Harold JohnsonDomestic extremists return to the Capitol GOP senator: Buying Treasury bonds 'foolish' amid standoff over debt ceiling, taxes Internal poll shows Barnes with 29-point lead in Wisconsin Democratic Senate primary MORE (R-Wis.), according to a new poll released Wednesday.
Feingold has 49 percent support to Johnson's 43 percent, the Marquette University Law School survey found.
Last month's edition of the same poll showed Feingold leading by 7 points.
In a three-way race with Libertarian Party candidate Phil Anderson included, Feingold's lead grows to 9 points — 47 percent to 38 percent, with Anderson at 7 percent.
Feingold, a former senator who lost his seat to Johnson in 2010, has 44 percent favorable and 36 percent unfavorable ratings.
Johnson fared worse, with 34 percent favorable and 32 percent unfavorable ratings.
In response to the survey, Johnson's campaign said Wisconsin voters had previously rejected Feingold over his ties to the political establishment, and that it was still early in the election cycle.
"Not much has changed, but what's clear is Ron Johnson is an outsider running his own race," spokesman Brian Reisinger said in a statement.
"As Wisconsinites tune into this campaign, they'll remember Senator Feingold is a career politician who doesn't know the first thing about creating jobs or keeping us safe — and after 34 years, they'll fire him again."
The Marquette University Law School conducted its poll of 805 registered voters in Wisconsin via cellphone and landline telephone interviews from Aug. 4 to 7. It has a margin of error of 4.6 percentage points.
Johnson is widely considered one of the most vulnerable GOP senators in the Democrats’ quest to take control of the Senate.
The party can reclaim control of the chamber if it flips five seats — or four, if Democrats keep the White House.
Johnson faces an uphill battle against Feingold, despite previously defeating him in 2010 amid a national backlash against ObamaCare.
The Badger State went blue for President Obama during both the 2008 and 2012 presidential election cycles.
— Updated at 5:36 p.m.