If you need to create a 100,000 spread between two candidates you can produce 100,000 fake ballots.

But if you have the opportunity to either alter ballots or miscount candidates from one of the races then you get a 2x leverage. Each ballot altered subtracts one vote from one candidate and adds one vote to the opponent. So if you need 100,000 votes you only need to alter or miscount 50,000 ballots.

Note that this will tend to preserve the down-ballot victories for one party while stealing the election for the other party in just one of the races on the ballot.