To win a presidential election, Democrats don't need 10 million illegal aliens to vote. They simply need them living in the U.S. and counted in the U.S. census. Electoral votes and Congressional seats are apportioned by population, not registered voters. This is why California has 53 seats in the House and 55 electoral votes, even though it probably has some Congressional districts where most of the residents can't vote.
Allocations and apportionment argument aside, those in the districts still have to get out and vote to keep outnumbering those voters who are sane. In the end, it is the amount of votes cast wherever that make the difference.