The Wisconsin Democrat supported a filibuster, giving more leverage to Scott Brown, who used his position to weaken the bill:

This is where Senator Feingold's principles come into play. Because he refused to vote for cloture on the Dodd-Frank bill without major revisions, a coalition of 60 liberal votes became impossible. So Senator 61, Scott Brown, became the dealmaker.
Since winning Edward Kennedy's Senate seat in a special election, Brown has certainly proved to be no conservative firebrand. Yet he has clearly staked out a position to the right of Collins and Snowe, especially on economic matters. So his vote on financial reform was a much tougher "get" than either of the Maine senators.
As it turns out, there were real consequences of Feingold forcing Brown into the pivot position. One of the provisions to come out of the House-Senate conference was a levy on large financial firms to pay for the costs of financial regulation. This provision was quickly dubbed a "bank tax". As a result, Brown, who had supported the earlier Senate version, began to waver. The provision not only ran counter to his ideological opposition to anything resembling a tax increase, but would have been costly to large financial firms in Brown's home state.
In the aftermath of Byrd's death, a defection by Brown would necessitate picking up both Democrats who had opposed the original Senate bill, Feingold and Washington's Maria Cantwell. Cantwell came around, Feingold didn't, and the bank tax was gone. As a result, $19 billion in costs were shifted from the banks to the taxpayer. Feingold has performed the legislative equivalent of voting for Nader in Florida in the 2000 presidential election: standing on principle only to get an outcome he couldn't possibly have wanted.

The frustration of compromising to satisfy a Senate supermajority has caused a lot of liberals to retreat into a fantasy world where withholding support can force the 60th Senate vote to move left. It just doesn't work that way.