The 30-second ad buy called "Billionaires" asks why the "out-of-state oil billionaires" are in New Hampshire.
The green group spent $200,000 earlier this year on ads hitting Brown, who is running against Democratic incumbent Sen. Jeanne Shaheen.
This latest ad comes to roughly $350,000, the group said.
“It’s no surprise that the out-of-state oil billionaire Koch Brothers are behind a multimillion dollar ad campaign to defeat Jeanne Shaheen and send one of Big Oil’s best friends back to the Senate. They know Scott Brown will continue where he left off by protecting oil industry profits at the expense of taxpayers and public health,” said Daniel J. Weiss, senior vice president for campaigns at the LCV.
Democrats have sought to tie GOP candidates to the Koch brothers, and Senate Majority Leader Harry Reid has blamed the billionaires for the ongoing stalemate in the Senate on a number of bills, including an energy efficiency bill that collapsed in May.
In all, the LCV expects to spend the most it ever has on campaigns on midterm elections this year.