First, there's voter impersonation--where you go to a precinct, and claim to be someone else and vote under their name. This tends to be relatively easy to catch; the general estimate of fradulent votes cast in this manner is less than 1 in a million, although there's certainly been more attempts that were easily caught (quite a few people last November tried to do voter impersonation to prove how easy it was and got busted instead). It can't be decisive, because before you get to a margin of victory in this range, you're going to discover that we can't actually ascertain the voting intentions that well (misread ballots happen about ~1 in 10000)--this is basically what happened in 2000; the margin of victory in Florida pretty much depends on what you consider constitutes an actual vote in the ballot.
A related type of fraud is using the vote of someone who should be ineligible to vote. You'll sometimes hear claims of millions of people on the voter rolls who shouldn't be on them, and the people who are concerned about this worry that they will be used for voter impersonation. Most of these cases basically boil down to the state didn't hear that you died or moved. States generally only bother to cross-reference their databases to purge people from these lists every few years, but it's still pretty easy to tell if someone cast a fraudulent vote in the meantime if you compare dates.
The least common case of voter fraud (at least in the US) is outright ballot-stuffing: where precincts lie about the counted results. Most precincts count their results in the presence of observers from independent election monitoring groups and representatives of candidates and political parties, which reduces the scope to commit this kind of fraud.
More common is mail vote fraud, where someone intercepts the votes of people who are voting by mail-in ballot. Estimates of this kind of fraud is perhaps 1 in 100,000--it's still not going to be decisive, since (as said above) the inherent inaccuracy in vote counting will crop up before then.
It's worth point out that there is no national vote in the US. Voting is entirely decided by the states, and many details about how you conduct the vote and vote counting process may be left up to individual counties or sometimes even precincts.
First, there's voter impersonation--where you go to a precinct, and claim to be someone else and vote under their name. This tends to be relatively easy to catch; the general estimate of fradulent votes cast in this manner is less than 1 in a million, although there's certainly been more attempts that were easily caught (quite a few people last November tried to do voter impersonation to prove how easy it was and got busted instead). It can't be decisive, because before you get to a margin of victory in this range, you're going to discover that we can't actually ascertain the voting intentions that well (misread ballots happen about ~1 in 10000)--this is basically what happened in 2000; the margin of victory in Florida pretty much depends on what you consider constitutes an actual vote in the ballot.
A related type of fraud is using the vote of someone who should be ineligible to vote. You'll sometimes hear claims of millions of people on the voter rolls who shouldn't be on them, and the people who are concerned about this worry that they will be used for voter impersonation. Most of these cases basically boil down to the state didn't hear that you died or moved. States generally only bother to cross-reference their databases to purge people from these lists every few years, but it's still pretty easy to tell if someone cast a fraudulent vote in the meantime if you compare dates.
The least common case of voter fraud (at least in the US) is outright ballot-stuffing: where precincts lie about the counted results. Most precincts count their results in the presence of observers from independent election monitoring groups and representatives of candidates and political parties, which reduces the scope to commit this kind of fraud.
More common is mail vote fraud, where someone intercepts the votes of people who are voting by mail-in ballot. Estimates of this kind of fraud is perhaps 1 in 100,000--it's still not going to be decisive, since (as said above) the inherent inaccuracy in vote counting will crop up before then.
It's worth point out that there is no national vote in the US. Voting is entirely decided by the states, and many details about how you conduct the vote and vote counting process may be left up to individual counties or sometimes even precincts.