They can steal about 1% of the vote—after that the logistics get too difficult.
(That is how it works in my home state of CT. They wait until the last minute to count the big city vote. If the statewide race is within 1% the Republican is toast—if more than 1% the Republican wins—has happened several times.)
That's how it works in my home state of MI, as well. Wayne County (Detroit) is always the last to report, and often hours after every other county in the state has reported.
Unfortunately for them, they couldn't fabricate enough votes in 2016 to give the election to sHrillary. This year they have many more mail-in ballots that will not be counted until after the polls close, so we won't get a number out of Wayne County for several days.
That is how it works in my home state of CT. They wait until the last minute to count the big city vote. If the statewide race is within 1% the Republican is toast—if more than 1% the Republican wins—has happened several times.
Somebody should do a statistical study of the percentage of close races where the Democrat wins versus where the Republican wins. If elections are fair it should be 50-50.