Calculating the math of the Electoral College, there are two basic rules heading into the November 2 election: If Senator Kerry is to become president, he must win Pennsylvania’s 21 electoral votes. Likewise, President Bush’s hope for reelection will almost certainly rest on his ability to win Florida’s 27 electoral votes. The outcome in both of these states remains in doubt, though Mr. Bush appears to have the edge in Florida and Mr. Kerry the upper hand in Pennsylvania.

Assuming each candidate clears these initial electoral hurdles, the battle for the presidency will move straight to the heart of the Midwest. The following three states will play a decisive role in determining who wins the White House:

OHIO (20 Electoral Votes): Although Ohio historically tends to vote slightly more Republican than the national average (Mr. Bush won Ohio in 2000 by 3.5 points, while just losing the national popular vote), the loss of manufacturing jobs has made it one of the toughest states for the president to hold.

Democrats recognized the potential to steal a win in Ohio and over the course of the last year the national party and liberal-leaning 527 organizations have invested tens of millions of dollars in a drive to register new voters – close to 800,000 by some recent estimates. Mr. Kerry’s schedule also tells of the state’s importance: he has visited Ohio more than any other state – a whopping 25 times since the beginning of March. This week Governor Schwarzenegger will be in Ohio campaigning on behalf of the president, as both sides pull out all the stops during the final days of the race.

As things currently stand, Ohio could not be any more of a dead heat. Mr. Kerry holds a slight edge (0.2%) over Mr. Bush in the RealClearPolitics Average, but of the five polls released in the past seven days, two show Mr. Bush slightly ahead, two give Mr. Kerry a small lead, and one has the two candidates tied.

WISCONSIN (10 Electoral Votes): In 2000, President Bush lost the state of Wisconsin by only 5,708 votes out of a total of 2.6 million ballots cast. This year the president is again running well in the Badger State thanks to a strong state economy and some help from his opponent: Mr. Kerry recently committed a blunder by referring to the Green Bay Packers’ stadium as “Lambert Field” (instead of “Lambeau Field”). Still, the race in Wisconsin is exceedingly close at the moment. The last four polls show Mr. Bush averaging only a 1.5% lead over Mr. Kerry – well within the margin of error.

IOWA (7 Electoral Votes): Like Wisconsin, Iowa went to Mr. Gore in 2000 by the slimmest of margins. Since 1980, Iowa and Wisconsin have voted at the national level with striking similarity and all indications are that it will happen again. Mr. Kerry currently holds a slight lead (0.7% on average) over Mr. Bush in the most recent round of polls. However, taken individually, the poll results range from a 4-point Kerry lead to a 2-point lead for Mr. Bush.

Wisconsin and Iowa, along with Minnesota, are traditionally Democratic states that have been trending Republican. Michael Dukakis won all three easily while losing to the president’s father by 8 points in 1988. Cultural and value issues in these Midwest states, along with a smaller-than-average minority populations, are working to make them more competitive for Republicans.

Because of reapportionment of the Electoral College votes this year, Mr. Bush sits in a slightly better position than Mr. Kerry. Mr. Bush could conceivably lose Ohio’s 20 electoral votes and still make his way to 270 (the number needed to win) by winning Wisconsin and either Iowa or New Mexico, or by holding onto New Hampshire and winning the one electoral vote at stake in Maine’s 2nd congressional district.

Mr. Kerry, on the other hand, has no such margin for error. Should he fail to win Ohio, it becomes almost impossible for him to collect an electoral vote majority. If there are no surprises in Florida or Pennsylvania on election night, the battle for the presidency will be won or lost in the heart of the Midwest.

Tom Bevan and John McIntyre are co-founders of RealClearPolitics.com