The excerpt below is from Wisconsin Wave; I hope you'll read the whole article, 'cuz it's a great summary of what happened in 2004.
But since I like to include visuals, I dug up the map at right. It was created in April, 2005 based on the exit poll data available at that time (click on the image for a larger version) and shows a "red shift" from the percentages reported by voters in the exit polls, which strongly favored Kerry by nearly five full points, to the percentages of the official, tabulated vote.
Per Richard Charnin, "[t]he unadjusted 2004 exit polls (state and national) were not released until about a year ago" (i.e., ca. June, 2011). According to the exit polls, out of the total respondents, 51.7% reported voting for Kerry and 47.0% reported voting for Bush (with the remainder voting for "Other"). Per the actual vote as tabulated, the final percentages were 50.7% for Bush vs. 48.3% for Kerry. This means the results shifted from Kerry being ahead by 4.7 points in the exit polls, to Bush being ahead by 2.4 points in the votes as tabulated, for a net, actual red-shift from the exit polls to the reported vote totals of 7.1 points. Can't believe you didn't hear more about it? Neither can I.
ROBERT F KENNEDY JR: The 2004 Presidential election was stolen via institutional fraud * * * * *(Emphasis supplied.) Footnotes and much more at the link. The issue of election integrity is, i.m.h.o., among the most under- and mis-reported of our time.
The first indication that something was gravely amiss on November 2nd, 2004, was the inexplicable discrepancies between exit polls and actual vote counts. Polls in thirty states weren't just off the mark – they deviated to an extent that cannot be accounted for by their margin of error. In all but four states, the discrepancy favored President Bush.(16)
Over the past decades, exit polling has evolved into an exact science. Indeed, among pollsters and statisticians, such surveys are thought to be the most reliable. Unlike pre-election polls, in which voters are asked to predict their own behavior at some point in the future, exit polls ask voters leaving the voting booth to report an action they just executed. The results are exquisitely accurate: exit polls in Germany, for example, have never missed the mark by more than three-tenths of one percent.(17) ''Exit polls are almost never wrong,'' Dick Morris, a political consultant who has worked for both Republicans and Democrats, noted after the 2004 vote. Such surveys are ''so reliable,'' he added, ''that they are used as guides to the relative honesty of elections in Third World countries.''(18) In 2003, vote tampering revealed by exit polling in the Republic of Georgia forced Eduard Shevardnadze to step down.(19) And in November 2004, exit polling in the Ukraine -- paid for by the Bush administration -- exposed election fraud that denied Viktor Yushchenko the presidency.(20)
But that same month, when exit polls revealed disturbing disparities in the U.S. election, the six media organizations that had commissioned the survey treated its very existence as an embarrassment. Instead of treating the discrepancies as a story meriting investigation, the networks scrubbed the offending results from their Web sites and substituted them with ''corrected'' numbers that had been weighted, retroactively, to match the official vote count. Rather than finding fault with the election results, the mainstream media preferred to dismiss the polls as flawed.(21)
UPDATE: Bob Fitrakis has another great article on vote rigging, at The Free Press.