U.S. Senate Democrats hoping to pass a historic overhaul of financial regulations lost a possible Republican supporter on Wednesday but the measure was still on track to clear Congress by the end of the week.

Republican Senator Charles Grassley, who has backed an earlier version of the bill, said he would not vote for the final version because he did not like changes that were made to cover its costs, oversee credit-rating agencies and regulate derivatives.

"It's a bill that most of Wall Street wants passed. And that's the last thing Iowans expect in any real reform bill," the senator from Iowa said in a prepared statement.

Democrats were still expected to get the 60 votes they need in a Thursday-morning procedural vote to advance the legislation, thanks to the support of three of Grassley's fellow Republicans.

Final congressional approval of the bill could come soon after, and President Barack Obama has said he wants to sign it into law next week.