The Karl Rove theory held that the way to win elections was to turn out your base: Rally the party faithful, muster them to the polls, and overwhelm the enemy. It worked for the Republicans in 2004 -- but only barely. Moreover, victory came at a cost: a bitterly divided electorate.

The base strategy did not work for the GOP in 2006. Instead, swing voters came back and swung the election. In the 2006 nationwide congressional vote, independents went 59 percent Democratic to 37 percent Republican, according to the exit polls. That was the biggest margin for either party measured among independents since the first exit polls in 1976.

Will the base strategy work this time? It seems unlikely. With fewer than 30 percent of voters now calling themselves Republicans, mustering that army won't overwhelm anything.

The parties' national conventions are over, and they did what they were supposed to do. They rallied the party faithful. Despite the split in the Democratic primaries, more than 90 percent of Democrats are lining up for presidential nominee Barack Obama, according to CNN polls taken by Opinion Research. That accounts for 30 percent of the vote. Republicans are nearly unanimous for their presidential nominee, John McCain. That's another 30 percent of the vote.

That leaves a lot of independent voters in the middle. The power to determine the outcome is in their hands. So whom do independents favor? They haven't made up their minds. Independents nationwide are split almost evenly between Obama (48 percent) and McCain (45 percent).

Independents want change. They give President Bush very low marks (a 22 percent job-approval rating). They think that Obama is more likely than McCain to unite the country, reform the system, and bring about change. But independents think that McCain is a stronger leader, who shares their values and has demonstrated an ability to work with both political parties.

McCain and Obama are competing for the independent vote by stressing similar themes and values. Both call themselves change candidates. Both promise to unify the country. The national polls show that neither holds a decisive edge. That means that the national polls are not likely to be much help this year in forecasting the outcome. Everything will depend on the outcome in the dozen or so battleground states. And Obama seems to be doing slightly better among those.

Seven battleground states have been polled since the conventions. In five of them, the party that carried the state in 2004 still has the edge, although in most cases by statistically insignificant margins. The Republican ticket is slightly ahead in Virginia and Missouri (by 4 or 5 points in CNN polls, compared with Republican margins of 7 to 9 points in 2004). In Michigan, Minnesota, and New Hampshire -- all states that went for Democrat John Kerry by 1 to 3 points in 2004, the Democratic margins are larger this time (4 to 12 points).

Two battleground states appear to be switching from Republican in 2004 to Democratic this time. One is Iowa, which went for Bush by 1 point in 2004 but gives Obama a 12-to-15-point lead in current polls. Iowa is where Obama made his breakthrough in the Democratic caucuses, and he retains a deep and loyal organization there. McCain, on the other hand, has never been competitive in Iowa. He came in fifth in the 2000 Iowa Republican caucuses and fourth in 2004.

Ohio is even more significant. Republicans have never won the White House without carrying the Buckeye State. It went for Bush by a 2-point margin in 2004. This month's polls by CNN and Quinnipiac University show the Democratic ticket with a narrow lead in Ohio (2 and 5 points, respectively). The state is struggling with a battered economy and a severe loss of manufacturing jobs. If the Democrats had carried Ohio in 2004, Kerry would have won the presidency.

In all of the battleground states, independents will determine the outcome. In Iowa, Michigan, Minnesota, New Hampshire, and Ohio, independents favor Obama. Those states are now tilting to the Democrat. Independents in Missouri and Virginia favor McCain. The Republican has the edge in those two states.

Unlike 2004, 2008 looks like a battle for the center -- and a battle in which the center differs from state to state.

This article appeared in the Saturday, September 20, 2008 edition of National Journal.

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