The 2004 Ohio election (first afaik to use electronic voting machines after the 2000 debacle) was straight up rigged, which directly led to Bush winning. There's no "smoking gun" but the difference between exit polls and official results indicates that votes were somehow flipped. This is not the only case where major discrepancies exist between exit polls and "official" results. There are many other cases of this happening, including in the 2016 and 2020 Dem primary.

EDIT: Here's a great article on the topic:

The first major indication that serious voter fraud had been committed was when the wide unexplainable discrepancies began to appear between the exit polls and actual vote counts and they all favored Bush.

Experts say exit polling is the most reliable polling because unlike pre-election polls, in which voters are asked to predict future behavior, exit polls interview people leaving the voting box about an act that they just completed.

On the basis of exit polls in 2004, CNN predicted that Kerry would defeat Bush in Ohio by a margin of 4.2%, but in the end Bush supposedly won Ohio by 2.5%.

In fact, precincts where Bush received at least 80% of the vote, the exit polls were off by an average of 10%, a pattern that experts say indicates Republican election officials stuffed the ballot box in those precincts.

Bush also tallied 6.5% more votes than the polls had predicted in Pennsylvania, and 4.9% more in Florida. According to Steven F Freeman, a visiting scholar at the University of Pennsylvania, who specializes in research methodology, the odds against all 3 of those shifts occurring in concert was one in 660,000.

“As much as we can say in sound science that something is impossible,” he says, “it is impossible that the discrepancies between predicted and actual vote count in the three critical battleground states of the 2004 election could have been due to chance or random error.”

Mr Freeman made a point of telling Robert Kennedy Jr in an interview for an article in Rolling Stone Magazine that he’s no Democrat lover. “I’m not even political — I despise the Democrats,” he said. “I’m a survey expert. I got into this because I was mystified about how the exit polls could have been so wrong.”

But Mr Freeman also said in Rolling Stone, “When you look at the numbers, there is a tremendous amount of data that supports the supposition of election fraud.”