The Senate’s Christmas Eve vote on health care reform reveals what appears to be a thermodynamic law of legislation: The longer the Senate is in session, the fewer bills it actually approves.

This odd corollary emerges from the annual Résumé of Congressional Activity published in the Congressional Record, a detailed tally produced by the Secretary of the Senate and the Clerk of the House of the number of days and hours each chamber was in session and the number of votes taken, measures introduced and nominees confirmed.

According to these annual tables, the Senate was in session for 191 days last year, the most since the 1995 government shutdown dragged the adjournment date past New Year’s Day. That session lasted 211 days. In 2007, the Senate was in session for 190 days, the only other year since 1995 with more than 173 days in session. In most other years during that period, the Senate has logged 135 to 165 days in session.

Despite the long hours, the marathon sessions are not producing historically large legislative outputs. According to the most recent résumé published in the Congressional Record, the Senate passed 478 measures in 2009, compared with 589 measures passed the prior year in 184 days and 621 measures passed in the 190-day session in 2007.

The most productive Senate sessions of the past 15 years were among the shortest: In 2000, the Senate passed 696 measures in 141 days; in 2004, it approved 663 measures in 133 legislative days; and in 2006, the Senate moved 635 bills in 138 days.

The first year of the 111th Congress was neither as long nor as fruitless in the House as it was in the Senate, as the House passed 966 measures in 159 legislative days.

There is no real correlation between time in session and volume of legislative activity in the House: In 1995, the House was in session for 168 days and passed only 483 bills; in 2007, the House met for 164 days and passed 1,127 bills. In 1997, the chamber passed 544 bills in 132 days, and in 2003, it passed 674 bills in 133 days.

The legislative numbers make no distinction between major pieces of legislation and post office naming bills or nonbinding resolutions.

A spokesman for Speaker Nancy Pelosi (D-Calif.) noted that the House moved several major pieces of legislation last year, including the Lilly Ledbetter Fair Pay Act, the Cash for Clunkers bill to prod motorists to buy new cars and the Credit Cardholders’ Bill of Rights Act, as well as the $800 billion stimulus package President Barack Obama signed last February. Democrats have widely argued that even with health care on hold, the 111th Congress has already been extraordinarily productive.

Overall, in 2009 Members of the House and Senate introduced 9,071 bills, passed a combined total of 1,444 and saw 119 enacted into law, according to the report. In the two years of the 110th Congress, Members introduced 14,042 measures, and 460 were enacted into law.

Jim Manley, spokesman for Senate Majority Leader Harry Reid (D-Nev.), said Republican obstruction is to blame for both the slow pace and low output of the first session of the 111th Congress. Manley pointed out that Reid had to file an unprecedented number of cloture motions simply to get bills to the floor, and that Republicans demanded cloture votes even on noncontroversial nominees.

“What I see is a Senator who went to great lengths last year to open up the floor much more than we had in the past to amendments, and the Republicans abused the process,— Manley said. “They have taken obstructionism to a whole new level.—

Minority Leader Mitch McConnell’s (R-Ky.) office declined several requests for comment on this story.

But Sen. Tom Coburn (Okla.), who has been a driving force behind many GOP procedural efforts to derail Reid’s agenda, said, “The reason we’re in session longer [some years] is a lack of consensus, and last year was probably the least consensus the Senate has had in a long time.—

Former Majority Leader Trent Lott (R-Miss.) said the Senate has gotten away from normal, family-friendly work hours where Senators and staff can expect to be home on weekends or after dinner, which wears on everyone’s ability to work together.

“The more you are in [session], the more days you are there, the less you get accomplished,— Lott said. “The Members get more obstinate, the minority digs in its heels.—

Lott also argued that Reid struggled because he was trying to push the Senate to meet a target he had set himself.

“Every time a Majority Leader decides he is going to make the Senate do something, it doesn’t work,— Lott said.

John Stanton contributed to this report.