In three of the nation's most hotly contested Senate races, the Republican-aligned American Crossroads GPS is airing new TV ads beginning today.

The group, co-founded by Karl Rove before the 2010 midterm elections, will spend $2.5 million attacking Democratic Sens. Sherrod Brown of Ohio, Jon Tester of Montana, and Democratic Senate candidate and former governor and Democratic National Committee chairman Tim Kaine of Virginia.

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The ads target the three Democrats for supporting major items on President Obama's first-term agenda. Kaine is lambasted for backing Obama's stimulus measure; Tester, for voting in favor of health reform; and Brown, for siding with Obama on health reform, cap and trade, and the Bush tax cuts.

Crossroads GPS is a 501(c)4 nonprofit group, meaning most of its money must be spent on issue advocacy. As such, it does not disclose the identities of its donors; its sister group, the American Crossroads super PAC, does.

The ads will run for 10 days. See them below: