Sen. Scott Brown (R-Mass.) is out with a new radio ad in which he praises his Democratic challenger Elizabeth Warren -- for following his lead.

The ad focuses on the People's Pledge, which is an agreement between the two candidates to keep third-party spending out of the race.

In the new spot, Brown takes credit for the pledge and praises Warren for signing on:

You've probably heard about the super PACs that have been created to influence elections, usually with negative ads. So I did something about it in my own race. I proposed and signed a pledge to stop third-party groups from coming into our state. The pledge is pretty simple: My campaign will pay a financial penalty if third-party ads are run supporting me, or opposing my opponent, and vice versa. No gimmicks, no fine print. I want to commend my likely opponent, Elizabeth Warren, for joining me in this People's Pledge. We both believe voters deserve a campaign where the candidates themselves make their case to the voters.

As the Boston Globe notes, the idea for the pledge really came from both sides: "Brown was first to call on Warren to denounce outside ads. Warren responded by calling on the sides to hammer out an enforceable agreement that would have consequences for the campaigns. Brown's campaign then drafted an actual agreement, which both sides signed after negotiations."

But as The Huffington Post's Ryan Grim reported, "the pledge Brown's side hammered out was authored by Sean Cairncross, according to the electronic signature that was still visible in the document's properties. Cairncross is the top lawyer for the National Republican Senatorial Committee, and the document names Cairncross' 'organization' as NRSC. He is a GOP campaign finance law expert who has been arguing to loosen restrictions for years."

According to a recent survey by the Western New England University Polling Institute, Brown is leading Warren by eight points.

Warren's campaign declined to comment.

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