Last week, the U.S. Senate failed for the first time in 48 years to pass an annual bill authorizing money for national defense - not over disagreement about the part of the bill that would repeal a ban against gays serving openly in the military but on procedural grounds. Moderate lawmakers inclined to support the bill balked Thursday when a vote was called what they considered to be too soon.

Before that, the Democrats who control the Senate failed in their efforts to stop filibusters on three other bills, including one that would provide long-term medical care for Ground Zero emergency workers who developed health problems after helping victims of the Sept. 11, 2001, terrorist attacks.

In a single afternoon, the Senate rebuked two constituencies revered by both parties: the military and the Sept. 11 rescuers.

The confounding actions left many in Washington to wonder whether this was an example of the dysfunction that increasingly seems to paralyze the Senate, the inevitable consequence of having a largely lockstep minority, or simply poor strategy by Senate Majority Leader Harry M. Reid (D-Nev.), who put some lawmakers in impossible binds. Or maybe all of the above.

An institution designed to chew over legislation slowly, refining and moderating bills passed by the House, now routinely chokes on them.

"Other than some exceptional moments, like health care, the Senate has a lot of trouble doing business in the modern era," said Julian Zelizer, a professor of history at Princeton University. "Partisanship, combined with the rules of the Senate, make for an institution that doesn't like . . . to act at all."

After spending months consumed by debates over health care and financial regulations, lawmakers left little time to address the litany of other issues before them, creating a backlog including the hot-button issues of tax cuts for the wealthy.

And the partisan tensions are flaring, with the Democratic majority humbled by an electoral rout in November and the Republican minority emboldened.

"It's an emotionally frayed body right now, which tends to exacerbate the worst instincts of the place," said Michael Meehan, a former top aide to Senate Democrats.

The events of last week illustrate a pair of seemingly contradictory trends, both brewing for decades. One is the hardening of the two parties. In the 1950s, famed Majority Leader Lyndon B. Johnson (D-Tex.) could attract support from moderate Republicans - which let him overcome defections of conservative Democrats.

Now, Republicans, at least, largely vote as one.

But at the same time that Republican and Democratic senators grew apart politically, they became even more reluctant to fight in the open. Senate history from the 1950s and 1960s is marked by long filibusters in which senators held forth for hours about things they disliked.