Actually, the numbers aren't 40/40. According to the most reliable, non-aligned (ie to one party or another, or one media conglomerate or another) US polling organization, The Pew Research Center, the numbers break down like this (only registered voters polled):

33% committed to Bush
38% committed to Kerry
29% swing voters

Of the swing voters, they break down like this:

13% favor Bush
10% favor Kerry
6% pure undecided

On the issues the punditry has declared "the theme of this election year" (there are two actually, the economy and the war on Iraq), swing voters are pessimistic about the economy, but support the war on Iraq and terrorism.

Here is their website summation of some other issues:

On two of three social and cultural issues that have played important roles in past presidential elections ­ abortion and gun control ­ swing voters fall somewhat closer to Kerry supporters than to Bush supporters. But on the social issue that looms large in this fall's contest ­ gay marriage ­ swing voters fall in between.

A majority of those who are committed to supporting Bush (56%) say they favor changing the laws to make it harder for a woman to obtain an abortion. By contrast, just 23% of committed Kerry supporters feel this way. Swing voters side with Kerry voters on this issue ­ just 33% favor toughening restrictions on abortion.

Similarly, a majority of Bush supporters (56%) say it is more important to protect the rights of gun owners than to control gun ownership. Only 26% of Kerry voters agree, and swing voters (at 37% agreement) fall closer to Kerry supporters than to Bush supporters.

On the issue of gay marriage, however, half or more of all three groups of voters say they oppose allowing gays and lesbians to marry legally. Bush voters are nearly united in opposition to gay marriage, with 82% opposed. Half of Kerry voters oppose gay marriage. Swing voters fall between these two groups, with 64% opposed ­ 18 percentage points less than Bush voters, 14 percentage points more than Kerry voters.