It will take me a little digging to find the source. The problem with the statistic is that "single" does include people who were once married, but no longer so.
Allow me to be helpful.
Here is an excel chart from the US Census Bureau on the 2004 election. As you can see, the numbers
are broken down into some of the sub-categories we might be interested in. As you can also see, the number of voters who have never been married, combined with the number who are currently divorced or separated, adds up to about 38.6 million, while the number who are currently married, or were married but are now widowed, adds up to about 86 million. These data emphatically refute the thesis that "the majority of voters are single".
