The two Republicans competing to unseat Democratic U.S. Sen. Tammy Baldwin this fall are locked in a tight race, according to a new Marquette University Law School poll.

For the first time, the poll shows state Sen. Leah Vukmir, R-Brookfield, with an edge over Delafield businessman Kevin Nicholson — but Vukmir's 2-point lead is well within the poll's 7-point margin of error for questions about the Republican primary. 

The race has tightened since February, when 28 percent of voters supported Nicholson and 19 percent backed Vukmir. Nicholson led Vukmir 37 percent to 32 percent in June, and Vukmir led with 34 percent to Nicholson's 32 percent in a poll conducted July 11-15. 

"The last two polls show that momentum has been with Leah ever since she earned the endorsement of the grassroots of the Republican Party of Wisconsin and now Speaker Ryan, Congressmen Sensenbrenner, Duffy and Grothman and the NRA," said Vukmir campaign manager Jess Ward in a statement.

Unlike the last Marquette poll, conducted in June, this month's poll did not present head-to-head matchups between the Republican candidates and Baldwin. In June, 50 percent of registered supported Baldwin over 39 percent who supported Nicholson. Against Vukmir, Baldwin netted 49 percent to Vukmir's 40 percent. The margin of error for those matchups was 4 points.

Thirty percent of Republican primary voters are undecided, which is unchanged from last month.

"We’ve known for months that this race would be tight. Establishment and Washington Insiders are circling the wagons for Leah Vukmir and it’s not surprising that the ballot is close," said Nicholson spokeswoman Ronica Cleary in a statement. "Now that we have released our first television ad today we’re geared up for the tough fight."

Baldwin's net favorability rating, which was slightly underwater last month, improved in July. In both June and July, 41 percent of voters said they viewed her favorably. In June, 43 percent viewed her unfavorably; that decreased to 40 percent in July. The margin of error was 4.1 points. 

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Jessie Opoien is the Capital Times' opinion editor. She joined the Cap Times in 2013, covering state government and politics for the bulk of her time as a reporter. She has also covered music, culture and education in Madison and Oshkosh.