WASHINGTON — The Senate Republican leader, Mitch McConnell of Kentucky, labored on Thursday to unite his caucus in opposition to legislation to tighten regulation of the nation’s financial system, as Democrats expressed dismay that Republicans were threatening not only to vote against the bill but to block the start of floor debate.

The Republicans, who just weeks ago lambasted the Democrats for negotiating provisions of the health care legislation behind closed doors, adopted an opposite approach and demanded that the Democrats agree to negotiate major changes to the bill before they would consider opening the regular process of floor debate and amendments.

It is a risky strategy in a high-stakes midterm election year, with public anger at Wall Street running high.

The Senate majority leader, Harry Reid of Nevada, has said he would like to bring the bill to the floor next week. President Obama at a meeting with Congressional leaders on Wednesday urged them to work quickly to reach a bipartisan accord.