A lot has been said about the gridlock in Washington, the political posturing and the partisan bickering and fighting. We all see it and watch it happen -- or should I say "not happen" as regards passing legislation in the Senate prior to the November elections. But the question is not often asked: Why does it happen?

What causes each side to line up at times as if in a kid's dodge ball fight and throw accusations at the other without either side venturing to the middle? I believe much has to do with the "numbers" behind the political scene.

Most voters in this country have become polarized during the past two decades. Roughly 80 percent of likely voters are split by party, and their opinions of politicians reflect this polarization. Self-identified Democratic voters increasingly cast ballots consistently and overwhelmingly for Democratic candidates, while self-identified Republicans do the same for GOP candidates at all levels. This polarization is highlighted in the job approval ratings of the presidents during the past 60 years when these ratings are broken down by party.

During the terms of Dwight Eisenhower, John F. Kennedy, Lyndon Johnson, Gerald Ford, Richard Nixon and Jimmy Carter, the average difference between the approval rating of voters from the president's own party and those from voters for the opposition was about 30 percentage points. Thus, for example, while voters from a president's own party gave him, say, a 70 percent positive job approval rating, the opposition party's voters gave the same president a positive job approval of 40 percent.

During the late stages of Richard Nixon's presidency, this approval difference moved up to an average of 40 percent, but in the presidencies of Carter and Ford, it returned to the 30 percent level.

It was under Ronald Reagan that the difference in approval ratings rose in a significant and consistent way. The difference between each party's approval ratings for Reagan was about 50 percentage points. During George H.W. Bush's term the average stayed at about the same 50 percent.

The size of this divide took another jump during Bill Clinton's presidency -- to an average of 58 percent -- nearly double the difference that had existed for most of the last four decades of the 20th century.

During the initial stages of President George W. Bush's presidency, this difference remained about 58 percent. But in the immediate aftermath of Sept. 11, 2001, and during Bush's handling of that crisis, the difference in job approval returned to 30 percent -- what it was 50 years ago. As time passed, however, and the country approached the midterm elections, the difference between the parties reasserted itself. Although Bush's overall approval ratings remained at historic highs, the difference between parties returned to the 50 percent level of the Reagan years.

Before Al Gore's announcement yesterday that he would not run for president in 2004, polling showed the unsettling prospect of a 71-point partisan difference in the voters' views of him.

What does all this mean? Politicians have a tendency to respond to their own constituencies and their base voters. If one party's base voters have a vastly different opinion of the other party's leader than of their own, clearly, the politicians' own positioning, their ability to compromise and their messages will reflect this reality.

President Bush has a 95 percent approval rating in his Republican base -- a phenomenal number. And his approval among independents stands at an unheard-of 75 percent. Yet his approval rating among the Democratic base is at roughly 40 percent to 45 percent. It's no wonder that the leaders of the Democratic Party are in no hurry to support the president and compromise on a consistent basis.

It's almost as if we are operating in two political worlds. Although overall the president is well liked and has strong approval ratings, the base vote of the opposition party does not see things the same way that the rest of the country does -- and Tom Daschle, Richard Gephardt, Nancy Pelosi and other prominent Democratic leaders find it difficult not to respond to their base. Hence, partisan bickering and posturing rise, and gridlock ensues.

Maybe with the presidency and the two branches of Congress in the same party's hands, we will be able to end some of this gridlock. The legislative successes immediately following the November election seemed to point in this direction, but the numbers are daunting.

The writer is senior adviser and strategist to the Republican National Committee.