Consumer activists in Massachusetts tried to present Scott Brown with a BMW on Thursday. They said it suited the senator better, given his pro-Wall Street stance, than the humble man-of-the-people pick-up truck that became an icon on the campaign trail.

With the House of Representatives having approved the Dodd-Frank regulatory reform bill on Wednesday night, the fate of the biggest Wall Street reforms since the 1930s now sits in the hands of the Senate and Mr. Brown, who has become an unpredictable swing vote in the chamber.

Mr. Brown became a Republican hero in January when he defeated a weak Democratic candidate for the Massachusetts Senate seat previously occupied by Ted Kennedy. He portrayed himself as an ordinary truck-driving Joe, with the mildly titillating claim to fame of having posed (almost) naked for Cosmopolitan magazine as a law student.

Now 50 years old, he has exerted a muscular influence on the Senate ever since his election, which stunned Democrats. The loss of the seat deprived the party of its 60-vote Senate supermajority and threatened to derail healthcare reform, though that was ultimately passed.

Immediately after the senator's election, Barack Obama, US president, announced a harsher approach to Wall Street - there would be a ban on banks' proprietary trading and they would have to sell off their hedge funds and private equity firms. The White House said this had been planned for weeks and was not a response to Mr. Brown's victory. A month later, Mr. Brown was branded a traitor by some conservative acolytes when he broke with his party to vote for a Democratic jobs bill. Then, in May, he joined three Republican moderates in voting for financial reform.

Most of his new critics on the left have forgotten that. Mr. Brown has spent the past few weeks picking the pockets of Chris Dodd, Democratic Senate banking committee chairman, to extract concessions desired by financial services companies in his state.

Boston-based State Street and Fidelity are big names in Massachusetts and the companies have sought changes, according to congressional aides working on reform. But the alterations, which loosen the very restrictions proposed by Mr. Obama after Mr. Brown's election, benefit a larger cadre of Wall Street interests.

Thanks to Mr. Brown, banks will be able to invest up to 3 per cent of their tier one capital in hedge funds and private equity firms. Paul Volcker, former Federal Reserve chairman, and Carl Levin, the Democratic senator from Michigan, who pushed for an outright ban, were aghast at the changes, according to people familiar with the talks. But they were prepared to swallow them to get the legislation through Congress.

The shock came when it transpired that Mr. Brown was still threatening to vote No, citing a $19 billion bank fee. Congress reconvened to remove it this week but Mr. Brown is holding out on a final decision during a week's recess. Elizabeth Weyant, a staff lawyer at the Massachusetts Public Interest Research Group that tried to deliver the BMW, is a milder critic than the stunt would suggest. "We were so grateful for his vote for the Senate passage of financial reform," she says. "I think he's going to do the right thing for Massachusetts consumers and support Wall Street reform."

Robert Byrd, the West Virginia Democratic senator whose death last week complicated the reform vote, once said: "Many new senators come here thinking that they will quickly make their mark on the institution. Soon, however, they learn that it is the institution that makes its mark on them; Mr. Brown has been playing hard politics and quickly making his mark.