Democratic U.S. Senate hopeful Elizabeth Warren has chosen a charity seeking treatments for autism to receive a donation from Massachusetts Sen. Scott Brown under an agreement the two signed to discourage outside political ads.

Brown responded by promising to write a $1,000 check to the Autism Consortium.

Under Jan. 23 pledge, each agreed to contribute 50 percent of the cost of a third-party ad to a charity named by the other candidate.

Brown promised to make a donation after learning that the Coalition of Americans for Political Equality posted online ads supporting him.

Under the terms of the deal, Brown was only on the hook for nearly $337.

The Republican said he decided to write a $1,000 check instead, describing himself as a longtime champion of autism research.