WASHINGTON — The sleepy United States senators thought they were done voting. But then, around 1 a.m. on the Saturday before Memorial Day, Senator Rand Paul, Republican of Kentucky and presidential candidate, marched spryly to the Senate floor to let it be known that, no, he would not agree to extend the federal government’s bulk collection of phone records program. Not even for one day.

With that, Senator Mitch McConnell, a fellow Kentucky Republican who only a few hours before was ebullient with the passage of a major trade package, was reduced to ordering his colleagues back to Washington next Sunday to try again to prevent the act from expiring.

The unexpected legislative collapse on the Senate floor, and Mr. McConnell’s morose departure, pointed up the quandary that has emerged since Republicans took control.

They have had successes, like passage of the hard-fought bill that could pave the way to the largest trade agreement in a generation and a bill to give Congress a voice in the Iran nuclear negotiations. And more senators are allowed to try to influence legislation through amendments, which Mr. McConnell’s Democratic predecessor as majority leader, Senator Harry Reid of Nevada, prevented.