In a close election — and 2004 has all the hallmarks of one — any group of voters can make the difference in the outcome. An exceptionally strong showing for one group, a lower-than-expected turnout for another, a change in historic voting patterns are among the many factors that can upset the predictions of the best forecasters.

This year, everyone is watching the undecideds. History tells us they usually break for the challenger. But I’m not paying much attention to them. The group is tiny, and their demographic profile suggests to me that a lot of them won’t show up at the polls on Election Day. Much better to look at groups whose size and history give them disproportionate electorate clout.

In 2000, voters who described themselves as Independents constituted 27 percent of the electorate, and they voted narrowly for Bush over Gore, 47 to 45 percent. Until recently, people who identified themselves as Independents voted for Republican presidential candidates. (1964 was an exception, when they, like virtually everyone else, voted for Lyndon Johnson.) But in 1992, Independents left their familiar mooring in the GOP and voted for Bill Clinton (38 percent) over George Bush and Ross Perot (30 percent each). Again in 1996, they cast their ballots for Clinton. In 2000, they returned (just barely) to the GOP fold. In the most recent Gallup, CNN, USA Today polls, Independents have been leaning toward John Kerry.

Catholics were 26 percent of the electorate in 2000. They voted narrowly for Gore over Bush (49 to 47 percent). In every election since the exit pollsters started collecting data on voters in 1976, Catholics have voted for the winning presidential candidate, which means that they, too, have no reliable party allegiance.

Kerry is the first Catholic major party nominee since John Kennedy, a fact that appears to have been lost on most voters and most Catholics. Just 26 percent in the Pew Research Center’s July poll knew that John Kerry was a Catholic. Forty-three percent of Catholics have made the connection. In a July 1960 Gallup survey, 84 percent identified John Kennedy as a Catholic. At this stage of the campaign, Catholics are tilting toward Kerry.

The pollsters call the third group I’m watching “some college.” It isn’t a very sexy label, but it is a significant one. “Some college” people are those who might have gone to a community college, a vocational school, or started college but dropped out. They are optimistic and upwardly mobile. Many of them live in the suburbs or beyond.

Politically they are important because their numbers are growing and because they swing back and forth between the parties in presidential and congressional elections. In 2000, they were nearly a third of the electorate. In 1980, 1984, and 1988, they voted for Republican presidential candidates. In 1992, 41 percent of them voted for Clinton, 37 percent for George H.W. Bush. Once again in 1996, they pulled the lever for Clinton. But in 2000, they voted George W. Bush. In Gallup polls this summer, they look like Bush voters.

Given their size and electoral sway, these groups will go a long way toward deciding who occupies the Oval Office next January.