WASHINGTON -- The Senate's senior Democrat and Republican reached a tentative agreement Thursday to impose modest limits on the filibuster, the delaying tactic that minority parties have long used to kill legislation.

The deal would reduce -- but not eliminate -- the number of times opponents can use filibusters on legislation and limit the time spent debating some bills and nominations. If acceptable to most lawmakers, the Senate was expected to approve the restrictions later Thursday.

The curbs on filibusters fall short of what Senate Majority Leader Harry Reid, D-Nev., initially said he favored months ago. He wanted to completely ban the tactic's use when the Senate tries to begin debating a measure, and he threatened to use Democrats' majority in the Senate to simply impose it.

That tactic is called the "nuclear option" because of the bitter partisan warfare it would likely trigger in the chamber, potentially halting almost any business the Senate tried to conduct.

The restrictions also fall far short of what some of the Senate's newer Democrats were demanding. Their proposals included requiring filibustering senators to actually debate on the chamber's floor, a practice immortalized in the film "Mr. Smith Goes to Washington" but seldom used in recent years.

Tight restraints on filibusters were championed by less-senior Democrats like Sens. Jeff Merkley, D-Ore., and Tom Udall, D-N.M, frustrated with the chamber's often glacial debates and the ability of the minority -- these days Republicans -- to kill bills with less than majority Senate support. It takes the votes of 60 of the 100 senators to halt the delaying tactics.

More veteran lawmakers like Sen. Carl Levin, D-Mich., were reluctant to stifle the tactic, mindful that his party could find itself in the minority after any election and would want to be able to use the maneuvers. But at least one long-serving Democrat expressed dissatisfaction, saying the pact would continue to let lawmakers in the minority force those in the majority to get 60 votes to prevail.

"It stands the principle of majority rule on its head," said Sen. Tom Harkin, D-Iowa.

Democrats say Republican use of the tactic has become almost routine and far too frequent. Republicans say they use it because Reid often blocks them from offering amendments.

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Associated Press Writer Jim Abrams contributed to this report.