A new group is looking to soften up Sen. Scott Brown's (R-Mass.) positive approval numbers.

MASSUNITING, a coalition of labor and Democratic-linked interest groups, tweaks Brown in a new ad for a fundraiser he is hosting at a minor league baseball game that includes a bobblehead of the senator for donors.

"Scott Brown promised to be an independent voice for Massachusetts," says the ad. "But instead, we got 'Bobble Head Brown,' saying yes to Wall Street and the national Republican agenda."

The group wouldn't divulge how much it is spending on the ad, but said it would run it on local television affiliates and cable during the evening newscasts for the next few days. A local Republican advertisement tracker said they had purchased roughly $24,000 in ads.

Brown's campaign pushed back on the ad. "The people of Massachusetts know that Scott Brown has been an independent voice in the U.S Senate fighting for jobs and working across the aisle to get our economy moving again," said Brown Campaign Manager Jim Barnett. "It's sad that the liberal special interest groups are engaging in negative attacks that distort the record. Scott Brown is going to continue to keep his focus on policies that will create jobs and put America back to work."

The ad is the latest in a string of shots liberal groups have taken at Brown, who has kept his job approval ratings above 50 percent in most recent polls even though he represents by far the most Democratic state of any Republican up for reelection next year. He also has a cash advantage, with almost $10 million in the bank for his reelection battle.

But Democrats and liberals believe that his support is soft and that by tying him to the Republican Party, which is highly unpopular in the state, they can drive down his poll numbers. Harvard Professor Elizabeth Warren, Democrats' preferred recruit, filed an exploratory committee with the Federal Election Comission last week and is expected to make a decision on the race sometime in early September.

Updated at 4:31 p.m. to include the estimated ad buy size and at 4:49 p.m. to include the Brown campaign's response.