Despite what some would argue was lackluster legislative output in 2008, the Democrats racked up big election gains in no small part by portraying the other side as obstructionists. That may be a tribute to the success of Pelosi in setting an agenda that fed the Democrats’ goals. With her power to control the House floor and a deft ability to assuage her party’s fiscally conservative Blue Dog Coalition, House Democrats in 2008 posted an all-time-high success rate on party unity votes, winning more than 93 percent of the time.

In addition, the high percentage of party unity votes in the House — 367 of 688 roll call votes this past year split the parties, at 53 percent the third-highest rate in the past decade — indicated that Republicans were usually not happy with the agenda set by Pelosi and her leadership team.

But partisanship didn’t always lead to legislative success for the Democrats. The Senate often couldn’t follow the House’s lead because Majority Leader Harry Reid of Nevada had a razor-thin working majority of 51-49 in a chamber where procedural rules benefit a minority that can hang together. For Reid, the biggest hurdle was finding 60 votes to invoke cloture and break a filibuster. Time and again that proved impossible.

A typical case arose over a House-passed bill to expand wage discrimination protections for workers. The bill aimed to overturn a Supreme Court decision that had prevented a woman from suing her employer because she had waited too long to file suit. House Democrats were almost entirely unified behind the bill; only six of them defected when the chamber passed it in 2007. Republicans, who objected that the bill would allow lawsuits for unintentional discrimination, were similarly united and voted against it, 2-193. That partisan split was mirrored in the Senate when Reid tried to bring the measure to the floor in April of this year. Only six Republicans joined the Democrats in voting to invoke cloture, and 41 voted no, just enough to block action. Reid cast the sole Democratic “no” vote — a parliamentary maneuver that preserved his right to try again later to revive the legislation, which never happened.

Leading Scorers, Party Unity: Click here to view chart

Another instance came in July, when House Democrats moved legislation to permit the Food and Drug Administration to regulate tobacco products. While the Republican caucus split, a narrow majority opposed the bill. And then, stiff GOP resistance in the Senate prevented further consideration.

Even the last vote of the session last week reflected how partisan splits were more inhibiting to legislative action in the Senate. On a mostly party-line vote, the House passed a bill, brokered between Democrats and the White House, to give emergency loans to U.S. automakers. One day later in the Senate, Reid could not persuade a sufficient number of Republicans to join the effort to advance the measure, and Congress quit for the year, leaving it languishing.

Party Unity Background: Click here to view chart

In the face of tough odds, Senate Democrats were victorious on more than 54 percent of party unity votes in 2008. But in historical terms, that was hardly impressive. Reid had prevailed on more than 67 percent of the votes that divided the parties the previous year, and Senate Republicans posted bigger victory margins every year between 1995 and 2006 — and in two of those years the Democrats had a nominal majority.

With a larger caucus in 2009, perhaps as many as 59, Reid may have more success if the parties remain divided. Several Republicans who often side with Democrats on critical votes will be back, notably Olympia J. Snowe and Susan Collins of Maine, Arlen Specter of Pennsylvania, and George V. Voinovich of Ohio. Snowe voted with the Democrats 61 percent of the time on party unity votes this past year, and Collins was a party defector only slightly less often, at 54 percent. Specter and Voinovich both sided with the Democrats more than 30 percent of the time. (In the most recent sign of their willingness to leave the GOP fold, all four voted with the Democrats on the auto loan measure.)

Roping the Strays

Hands down among congressional leaders, Pelosi had the greatest success in winning partisan votes during the past year by exploiting GOP defectors and at the same time holding the sometimes fractious Democratic caucus in line.

Party Unity History: Click here to view chart

In several cases, Republicans who were potentially vulnerable to defeat backed away from their party label and became frequent, if not reliable, Democratic allies. Florida Republican Ileana Ros-Lehtinen , for example, moved steadily away from her party’s leaders, and her party unity score in 2008 was 18 percentage points lower than it was for the entire eight years of the Bush presidency. Two other Florida Republicans similarly abandoned their party more often. Mario Diaz-Balart ’s party unity score was 9 percentage points lower in 2008 than it was during the Bush presidency overall, while his brother Lincoln Diaz-Balart ’s was 8 points lower. All survived serious challenges in November.

Not all Republican defectors were able to capitalize on their independence. Wayne T. Gilchrest of Maryland has long had one of his party’s lowest party unity scores, and his ideological moderation was why he lost his bid for a 10th term — to a much more conservative Republican in the party primary. And Christopher Shays of Connecticut, who voted frequently with Democrats since he arrived in 1987 and had the fourth-lowest party unity score in the House GOP this year, lost in November mainly because he stuck with his party on one unpopular top-tier issue, the Iraq War, and broke with it on another, the financial industry bailout.

Vote Charts: Click here to view chart

While some Republicans were abandoning their party, Pelosi encouraged partisan discipline from the most fiscally conservative in her ranks by sticking mostly to her pledge to pay for new spending increases or tax cuts with revenue raisers that would prevent the budget deficit from growing.

Blue Dogs Gene Taylor of Mississippi, Robert E. “Bud” Cramer of Alabama, Collin C. Peterson of Minnesota and Dan Boren of Oklahoma all posted unity scores that were higher by 17 percentage points in 2008 than for the Bush era as a whole. Overall, Blue Dogs voted the party line on average 91 percent of the time, barely less often than all House Democrats.

“The leadership allowed us the opportunity to make our case for fiscal responsibility,” said Charlie Melancon of Louisiana, who will be co-chairman of the Blue Dog caucus in the 111th Congress.

Comparative Comity

The level of partisanship seen this year — and actually during the past decade or two — hasn’t always been the pattern in the post-World War II period. Votes that divided the parties tended to be less frequent, and regional or other differences often pitted Republicans or Democrats against themselves instead of against each other.

That did occur on rare occasions in 2008. House Republicans were at times internally divided over reauthorizing farm programs, combatting AIDS overseas and the Bush administration’s request for authority to save faltering financial institutions. House Democrats disagreed among themselves over defense programs and international debt relief. And in the Senate, Republicans couldn’t agree on a Democratic bill to expand government-funded health care for children, while their Democratic counterparts struggled for unity on legislation granting Bush more authority to conduct surveillance of foreign intelligence targets communicating with people in the United States.

Such instances were far more common during the early years of CQ’s vote studies. In the decades before the Clinton administration, instances of party unity votes were fewer and average lawmaker unity scores were lower — typically in the 70 percent range and occasionally in the 60s. In a few years, they reached as high as 81 percent.

The current period of polarization began roughly in 1993, when Republicans and Democrats alike saw their average party unity scores jump into the mid-80s. That high level of partisanship accelerated with the GOP takeover of Congress after the 1994 elections and has continued unabated since.

The extent of the shift may be amplified by tighter floor control exercised by leaders of the majority party, said Jon R. Bond, a political scientist at Texas A&M University. “The majority party just won’t bring a vote up unless they know they are going to win,” he said. More telling, Bond said, the partisanship of today is a return to traditional American party politics, while the relative comity that existed from the 1950s to the 1980s was the exception.

“Even after all these years of increases in party voting, it’s still not nearly as high as it was in the 19th century,” he said.

Like Pelosi, Reid is promising more bipartisanship in 2009. “We know full well that this election was not a mandate for Democrats so much as it was a referendum for bipartisanship,” he said. And like Boehner, Senate Minority Leader Mitch McConnell of Kentucky is pledging to work with the Democrats, so long as they don’t overreach. “We face a simple choice: We can either work together to confront the big issues of the day that neither party is willing or able to tackle on its own, or the majority can instead focus on narrow, partisan issues that appeal to a tiny sliver of the populace,” he said.

Whether such pledges lead to a meeting of the minds, of course, will depend on whether Republicans and Democrats come to see bipartisanship as in their political interest and, if recent history is any guide, that is unlikely. In both the House and the Senate, Pelosi and Reid can take heart in election results that will allow them to pursue partisan agendas and potentially have greater success at passing laws.

In the House, Pelosi will continue to weigh the demands of her restive liberal wing against the fiscal concerns of Blue Dogs. As long as she keeps them all in line, she won’t need a single Republican vote. For his part, the Blue Dogs’ Melancon said he expects that the “extreme right and extreme left will flex their muscles,” but he said the Blue Dogs’ size, now about 50 members, ensures that their voice will be heard.

In the Senate, Reid has drawn a significantly stronger hand and is likely to be more successful because he will need to pick off only a few Republican moderates. He undoubtedly knows, as does McConnell, that when the Republicans had sizable Senate majorities during the Bill Clinton years, and during much of Ronald Reagan’s presidency, they had far more success than Reid has seen in the past two years. So McConnell will have to pick his battles carefully, assessing when he most wants to keep his moderates in line while convincing them that it’s in their political interest to stay there.