And though he kept his campaign promise to oppose the big health care legislation, and has voted with the Republican leadership in the overwhelming majority of cases, he has also sided with Democrats on some important issues. Most notably, he voted in favor of the financial regulatory bill even though the Senate Republican leader, Mitch McConnell of Kentucky, and the rest of the conference leadership, voted no and denounced the measure as overly expanding the government.

“Listen, I have always said I don’t work for Mitch McConnell and I don’t work for Harry Reid, I work for the people of Massachusetts,” Mr. Brown said in an interview. “I am not quite sure what all the surprise is, and people wondering kind of like, ‘wow, he’s independent.’ I have always been this way. I am going to look at each and every bill and look at the merits of it.”

But his willingness to team up with Democrats is already raising eyebrows and ire among some voters in Massachusetts who backed his candidacy. They say he is betraying some of the conservative ideals that he espoused during the campaign  especially in his vote on the financial regulation bill.

“People are disappointed in this vote, because we see it as contrary to limited government and supporting free markets,” said Christen Varley, the leader of the Greater Boston Tea Party. “I think people are realizing that in some situations Senator Brown will be an ally and in some he will not. Some people are angry. Some people are disappointed but willing to wait and see what happens with the next votes.”

Ms. Varley said she was urging Tea Party members to contact the senator’s office and express their disappointment but also counseling them to develop a healthy sense of pragmatism, especially in a state as traditionally liberal as Massachusetts.