Washington ? The members of the new Congress will be ushered into Washington to the standard accompaniment of pledges of mutual esteem and bipartisan cooperation. The Republican and Democratic floor leaders of the House and Senate — six of the eight are new to their positions — will vow to set aside their narrow agendas and reach out to each other to take care of the nation’s urgent needs.

Don’t bet on it.

Even with the best intentions, they would have to overcome powerful institutional forces that have been decades in the making and set aside their own well-established patterns of behavior to avoid two more years of partisan gridlock on many issues.

If the past is prelude on Capitol Hill, you can expect to see major policy disagreements between the parties in the run-up to the 2004 presidential and congressional elections, as Republicans support and Democrats resist key parts of President Bush’s agenda.

The reason for such a forecast is that party caucuses on both sides of the Capitol have become more cohesive internally and further apart from each other philosophically. More and more of the issues divide on partisan lines. And the new leaders in the House and Senate, like the old ones, reflect that growing gulf between the parties.

Reporters who have covered Congress over several decades know from their daily experience that almost each session, they have found fewer moderates or progressives on the Republican side and fewer conservatives among the Democrats — especially when it comes to fundamental economic and social policy questions and the role of government in American life.

That reality is summarized neatly in the “Party Unity” scores published last week by Congressional Quarterly, the private news research organization that has been monitoring Capitol Hill for more than half a century.

Each year, CQ counts the number of votes on which a majority of Republicans oppose the stand taken by a majority of Democrats. Then it calculates the percentage of times on which each member has voted with the party majority on those roll calls.

When I averaged the year-by-year results for both chambers, I found the percentage of partisan-divide roll calls has gone from 39 percent in the 1970s to 47 percent in the 1980s to 58 percent in the 1990s.

Even more striking is the growth in cohesion — call it discipline or philosophical agreement — within both party caucuses. In the 1970s, on the partisan roll calls, the average member of Congress backed the party position 65 percent of the time. In the 1980s, the average degree of partisan loyalty rose to 73 percent; and in the 1990s, to 81 percent. In these past two years, it has been 87 percent.

Another way of expressing that trend is this: In all of the 1970s, there were no years where the average Republican or Democrat voted “right” from a partisan view as much as 75 percent of the time. In the 1990s, by contrast, there was only one year in which the Republican average fell as low as 74 percent — and none in which the Democratic average was below 79 percent.

The new leadership teams in both parties reflect that intense partisanship. In the House last year, incoming Republican Majority Leader Tom DeLay of Texas and new Democratic Minority Leader Nancy Pelosi each supported the party position on these roll calls 99 percent of the time. That means they took opposite stands more than 200 times. DeLay’s deputy, Roy Blunt of Missouri, was a 98 percenter, while Pelosi’s whip, Steny Hoyer of Maryland, broke party ranks more often, giving him an 89 percent score.

In the Senate, the new majority leader, Bill Frist of Tennessee, often described as “a healer,” was every bit as loyal a partisan last year as the man he replaces, Trent Lott of Mississippi. Lott was a 98 percenter; Frist, 97 percent, and the new Republican whip, Mitch McConnell of Kentucky, also voted the party line 97 percent of the time.

The only surprise is on the Democratic side of the Senate. Democratic Leader Tom Daschle agreed with his party majority only 80 percent of the time — a remarkably low figure for a man often described by Republicans as a partisan obstructionist. His deputy, Harry Reid of Nevada, was more partisan, toeing the Democratic line 94 percent of the time.

Nothing in these long-term trends or in the personal records of the party leaders suggests that bipartisanship will be easily achieved in the 108th Congress. Fasten your seat belts. There’s turbulence ahead.

David Broder is a columnist for Washington Post Writers Group.