The Massachusetts senator said in a letter to five advocacy groups that bill "changes the rules in the middle of the game." He says it will give "a tactical and political advantage" to labor unions little more than 100 days before an election.
The bill is sponsored by New York Democratic Sen. Charles Schumer. He complains the high court itself changed key fundraising rules with its January decision easing restrictions that had barred corporations from paying for political ads.
Last week, Brown angered many in the GOP by saying he would support a financial regulatory bill.
