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Mr. Cunningham also said buyer’s remorse isn’t uncommon. He joked that if political donors asked for their money back every time they were disappointed, the banking system wouldn’t be able to handle all of the transactions.

Mr. Brown may have been relatively unknown early in his 2010 bid, but his victory boosted his national profile. He plans a national book tour for his autobiography, which is expected to be released soon, and will hold political fundraisers along the way.

An attorney for the Brown campaign recently asked federal election regulators for permission to use campaign cash to buy thousands of copies of Mr. Brown’s book to send supporters signed and unsigned copies as “thank you” gifts.

Mr. Wheeler is more interested in making sure the chapter on Mr. Brown’s Senate career is short.

“We would support someone to run against him in the primary,” Mr. Wheeler said, adding that the group’s early backing proved crucial for Mr. Brown in the special election. “If voters in Massachusetts had not seen how people across the nation rose up and supported him, he would not have been elected.”