It's time to throw out that old map with the red states and blue states. The map that implies that all but a handful of states will definitely vote Republican or Democratic and that the real contest will be decided in Florida or Ohio or whatever.

For a time, the map served its purpose. Only three states changed parties between the 2000 and the 2004 presidential elections, and the average change in percentage margin in those states was only 1.5 percent. But such hugely static political patterns are the exception rather than the rule in our history.

The last two presidential elections whose results so closely resembled each other were 1952 and 1956, when the two parties nominated the same candidates and only four states' results were different.

In 2000 and 2004, the Republicans nominated the same man and the Democrats nominated men with similar personas and similar places on the political spectrum.

This year, it's different. The Republicans will nominate John McCain, and the Democrats seem 95 percent certain to nominate Barack Obama. There are important differences between them and their parties' previous nominees. Many votes that went Democratic in 2000 and 2004 are available to McCain. Many votes that went Republican in 2000 and 2004 are available to Obama. And many of the new voters surging into the electorate may be available to both candidates.

Voters have a clear generic preference for the Democratic Party, but recent polls show a McCain-Obama race to be close. And don't be surprised if those numbers move around in the course of the campaign.

It's not like we haven't seen voters move around before. At the beginning of the 1990s, it was conventional wisdom that Republicans had a lock on the presidency and Democrats had a lock on Congress, or at least on the House of Representatives. After all, Republicans had won five of the last six presidential elections and Democrats had held control of the House for 36 years. Continued...