So lets say Obama wins. Popular vote is really close (1%ish), but he carries Ohio and and pulls the electoral lead.
Then a month later investigations turn up that the only reason he won was because of voter fraud. What then?
Assuming it's between the election and the time the Electoral College meets in mid-December, there is a chance to turn. Remember, the winner of each state sends delegates who at least "promise" to vote for their party candidate.
It is, in theory at least, possible to turn enough of those voters to where the winner of the Electoral College is thrown into doubt or even to McCain.
In reality, there have been "protest" EC votes--remember Reagan got 1 in 1976, Lloyd Bentsen got 1 in 1988, Walter Jones (Dem) got 1 in 1956, and John Hospers (Libertarian) got 1 in 1972. So it is possible that in fact enough "protest" votes could be thrown to Hillary or over to McCain to bring the House into the deal.
And frankly, if the Dems in the House ever want to see power again, they would do the right thing. To vote Obama in even in the face of voter fraud would kill their power for the forseeable future.