Sen. Scott Brown confronted his Senate-race opponent Friday in a voter-registration controversy, calling on rival Elizabeth Warren to reimburse the state of Massachusetts for a mailing that promises to bring more welfare recipients into the voting booth this fall.

Senator Brown charges that, by mailing voter registration forms to people on public assistance, the state is using taxpayer money for partisan purposes – with the aim of helping Ms. Warren win their close-fought race. The state recently agreed to mail the forms in response to a lawsuit backed by liberal groups, who say Massachusetts has done a poor job complying with a 1993 federal law requiring that people be able to register when they take actions like renewing a driver's license or signing up for welfare.

"It's been disturbing for a lot of people to learn that the state's welfare department undertook an unprecedented voter registration drive," said Brown, a Republican, in a statement released Friday. "It is clear that this was done to aid Elizabeth Warren's Senate campaign."

Citing Warren's $13 million campaign account, he said his Democratic opponent should "should immediately reimburse the state for the cost of this mailing and stop playing politics with the taxpayers' money."

Brown's call for that action is unusual, since Warren herself is not a party in the lawsuit. Brown said the state was acting "at the behest of Elizabeth Warren's daughter and the organization she represents," a reference to Demos, a key group behind the lawsuit. Brown did not provide evidence that Warren's daughter, Amelia Warren Tyagi, played a role in bringing the lawsuit.

However, Ms. Tyagi chairs the board of trustees of Demos, which represented plaintiffs in the suit. Demos President Miles Rapoport said this week that Tyagi didn't "encourage any work in order to benefit her mother's campaign."

In throwing down a verbal gauntlet to Warren, Brown is also taking the state's political establishment to task. Few states are so dominated by Democrats as Massachusetts, where both the governor and legislature are controlled by Warren's party.

In a fundraising note sent Thursday, Brown campaign manager Jim Barnett called the registration effort the "latest example of Beacon Hill machine politics," in which the state is targeting welfare recipients, known as likely to lean Democratic, "using hundreds of thousands of dollars in taxpayer money."

Mr. Barnett said the effort encompasses nearly half a million welfare recipients, or about one-third of the votes a candidate will need to win the election. Of course, not all the people who receive letters will register, vote, and make Warren their choice. But the Senate race is currently a tossup, with polls suggesting that a modest shift in the electorate could be important.

The Boston Globe reported Friday that, of five states where Demos has taken legal action regarding the so-called "motor voter" law, Massachusetts is the only one that agreed to send mass mailings to welfare recipients.

However, a spokesman for the Massachusetts Attorney General Martha Coakley says many states have agreed in settlements to reform their voter registration practices, or have faced costly litigation against Demos. Brown won his Senate seat in January 2010 after defeating Ms. Coakley, a Democrat, in a huge upset.

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Conservative observers say the timing of moves by Demos appears linked to political motives. Legal actions this year have come not just in Massachusetts, home to the important Brown-Warren Senate race, but also in Pennsylvania, considered a swing state in the presidential election.

In its defense, Demos says its track record of advocating stronger enforcement of the motor-voter law goes back a decade, and has included states that are neither presidential swing states nor ones with high-stakes Senate contests.