The warning has ricocheted around the financial world in recent weeks, in conversations at Midtown restaurants and Washington fund-raisers, carrying urgent appeals for money from financial executives around the Northeast: The battle to re-elect Senator Scott P. Brown, the Republican from Massachusetts, just got a little more interesting.

“Senator Brown is a free-market advocate who believes that our strength as a nation comes from the ingenuity and hard work of its people,” read an invitation to a fund-raiser at a New Canaan, Conn., country club last week, that circulated among hedge fund and private equity executives. His Democratic opponent, the invitation noted, was all but certain to be the financial industry’s most prominent foe: “big government liberal Elizabeth Warren.”

Mr. Brown, a freshman who harnessed populist Tea Party anger to win the seat once held by Edward M. Kennedy, has taken more money from the financial industry than almost any other senator: all told, more than $1 million during the last two years, according to data from the Center for Responsive Politics.

Of the 20 companies that accounted for the most campaign donations to Mr. Brown, about half were prominent investment or securities firms like Morgan Stanley, Fidelity Investments and Bain Capital. His donors include such blue-chip names as Gary Cohn, the president of Goldman Sachs, and the hedge fund kings John Paulson and Kenneth Griffin.