It would take far more than a modest dispersal of Democrats to make a congressional-district method of electoral vote allocation more advantageous to the Democrats than would be a winner-takes-all method. Remember, most big cities are in states that the Democrats carry in presidential elections, and anything that gives the Democrats less then 100% of the state’s EVs is sub-optimal for them. For example, if half of Detroit’s population moved to the suburbs, it could allow the Democrats to carry a couple more CDs, but only if the Democrat presidential candidate carries the state of Michigan as a whole; a CD method at a time in which half of Detroiters moved to the suburbs could yield a 9-8 Democrat advantage in EVs, which is better for them than the 10-7 GOP advantage in EVs had the 2004 elections featured the CD method in Michigan, but is still far worse for the Democrats than the 17-0 sweep they would get with a winner-takes-all system.
The only way that Democrats could benefit from a CD system is if they control the redistricting process in just about every multi-member state and adopt an effectively partisan redistricting plan in each such state. But even then it might not be enough. For example, the heavily partisan Democrat gerrymander of 2002 in Indiana eventually resulted in 3 additional Democrats being elected to Congress, but it did not increase the number of districts that voted for Gore or Kerry. Another example is GA, where the outrageous Democrat gerrymander of 2002 created 4 new districts that favored the Democrats on paper, but only two of those new districts actually voted for Gore or Kerry.
Bush wins by an even BIGGER margin in 2004 if the "electors split by district" plan was in place, because Bush won over a TON of marginally DemocRAT districts in the south and west.
2004 ELECTION RESULTS
CONGRESS
Republicans = 287 seats (House + Senate)
Democrats = 242 seats (House + Senate)
PRESIDENCY - WINNER TAKE ALL SYSTEM
Bush = 286 electoral votes
Kerry = 252 electoral votes
PRESIDENCY - EV PER CONGRESSIONAL DISTRICT
Bush = 314 districts/electoral votes
Kerry = 193 districts/electoral votes