The Wisconsin Senate race is a virtual toss-up, with Republican Sen. Ron Johnson edging Democratic challenger Mandela Barnes within the margin of error in a Marquette University Law School poll.

FOX NEWS POLL: POLARIZATION DEFINES THE MIDTERM ELECTION

Approximately 50% of likely voters said they support Johnson in the race, compared to 48% for Barnes, according to the Marquette poll of 679 likely voters. The margin of error for the poll's likely voters sample is plus or minus 4.8 percentage points.

Marquette Law School defines "likely voters" as those "who say they are certain to vote or who say they have already voted." 

Among the slightly larger sample of 802 registered voters, Johnson maintains 48% support versus Barnes' 45% support.

The Marquette Law Poll was conducted from Oct. 24 to Nov. 1 via landline and cell phone. The registered voter sample has a margin of error of plus or minus 4.6 percentage points.

MIDTERM STUDY FINDS MAINSTREAM MEDIA GAVE REPUBLICANS ‘87%’ MORE NEGATIVE COVERAGE THAN DEMOCRATS

Liberal groups in Wisconsin seeking to change rules governing absentee ballots suffered a blow this week after an appeals court and a circuit judge ruled that ballots cannot contain partial addresses of witnesses.

In his order, Judge Juan Colas wrote that Wisconsin elections have been conducted for 56 years without a legally binding definition of what constitutes a witness address on a ballot. He said clerks have been legally free to interpret the term in good faith, drawing on non-binding guidance from the Wisconsin Elections Commission, its predecessors and advice from attorneys.

Current guidance from the Wisconsin Elections Commission is that an address must include three elements: a street number, street name and municipality.

Fox News's Bradford Betz contributed to this report.