One of the things pundits are having a hard time figuring out is why the electorate seems to have come to a point where almost exactly half of people support the Democratic presidential candidate and half support the Republican. A reason why they are having so much trouble, I believe, is that the answer lies outside the presidential realm and results primarily from changes in Congress.

Poll after poll has consistently shown that the American people like gridlock. They don’t trust either party to run the entire government and like having one in a position to check the other. The most recent poll in December 2002 by NBC News and the Wall Street Journal found 62% of Americans in favor of Congress and the White House being controlled by different parties. Only 29% were in favor.

From 1932 to 1994, the Democrats basically had a lock on Congress. In my view, that is the key to why Republicans were so successful winning the presidency in the postwar period. Voters who couldn’t quite make up their minds felt comfortable casting their ballots for a Republican president most of the time because they knew if they made a mistake the Democrats would still be in a position to exercise restraint and oversight.

The foundation of the Democratic lock on Congress was gerrymandering – drawing congressional district lines in order to maximize the number of districts with a majority of Democrats in them. This was done by creating a few districts that were virtually 100% Republican.

Consider this example. Say you have a region with 2 million voters, half Republican and half Democrat, divided into four congressional districts of 500,000 voters. You create one district of 500,000 Republicans and divide the remaining Republicans among the other three districts. This leaves three districts in which the Democrats win easily by a two to one margin.

But to play this game, you have to have control of state legislatures because they draw the congressional lines after each decennial census. It was here that the Republicans had a severe disadvantage. According to the National Conference of State Legislatures, from 1956 to 1993, Democrats controlled a majority of state legislatures every year except one, when there was a tie. And in many years the Democratic majority was overwhelming. In 1974, they controlled both houses of the legislature in 37 states. Republicans had only four, and eight were divided.

In the last few years, however, Republicans have made great strides in the states. They now have majority control in the legislatures of 21. Democrats control 17 and 11 are split. This has allowed Republicans to finally play the gerrymandering game, as they have done so successfully in Texas, which is the main reason why most political observers expect the Republicans to maintain control of the House of Representatives. Also, having more legislative seats gives Republicans a deeper bench of candidates to run for congressional seats.

Another factor, of course, is the well-known propensity of incumbents to win re-election. Although the press attributes this mainly to their superior fund-raising and ability to steer pork barrel projects into their districts, I think inertia is of much greater importance. Thus, while the latest NBC/Wall Street Journal poll has the parties tied at 43% in terms of votes for Congress, people say they support re-electing their current congressman by a 49% to 34% margin.

What all this means is that as voters come to expect continued Republican control of Congress, those votes that automatically went to Republican presidential candidates in the past may go to Democrats in the future. I think if most voters were as certain of Republican control of Congress today as they were of Democratic control in the 1960s and 1970s, it might be enough to push the Democratic presidential candidate over the top. But it will probably take many more electoral cycles and years of Republican control of Congress before that happens.

For now, at least a few conservatives and libertarians who normally vote Republican are voting for Senator Kerry precisely in order to bring back gridlock. One is well-known Web blogger Andrew Sullivan, who thinks gridlock will keep government spending under control. “Divided government,” he says, “is perhaps the only real mechanism we have…to restrain the politicians in D.C. from spending even more of our money.”

According to economist Bill Niskanen of the Cato Institute, Mr. Sullivan has a point. His research shows that the rate of growth of federal spending is significantly lower when the White House and Congress are under control of different parties than when they are unified.

In coming years, the gridlock factor will tend to help Democratic presidential candidates as long as Republicans keep Congress. For now, it helps explain why the electorate is equally divided.

Mr. Bartlett is a senior fellow at the National Center for Policy Analysis.