WASHINGTON  Senate Republicans on Thursday rejected a bill to aid small businesses with expanded loan programs and tax breaks, in a procedural blockade that underscored how fiercely determined the party’s leaders are to deny Democrats any further legislative accomplishments ahead of November’s midterm elections.

The measure, championed by Senator Mary L. Landrieu, Democrat of Louisiana, had the backing of some of the Republican Party’s most reliable business allies, including the United States Chamber of Commerce and the National Federation of Independent Business. Several Republican lawmakers also helped write it.

But Republican leaders filibustered after fighting for days with Democrats over the number of amendments they would be able to offer. A last-ditch offer by Democrats to allow three was refused by the Republican leader, Mitch McConnell of Kentucky.

“The majority leader has graciously given us three amendments and what I’m saying is three amendments is not enough; he knows that,” Mr. McConnell said on the Senate floor. “We are not expecting to have an unlimited number of amendments, but three amendments will not suffice.”