I have no idea what the effects on the 2012 election would have been if all 50 states and DC had adopted that concept and it doesn't matter because both sides knew the rules going in. It does seem, though, that awarding Electors by CD would put more than eight or nine states in play every four years.
In California, which has 55 EVs, Romney would have had a shot at as many as 16 Electors in Districts likely to be represented by Republicans.
Similarly, in Texas Obama might have picked up as many as 12 of the 38 EVs available there.
The direct election of the President would likely mean that all of the attention would be paid to the high population areas: New York City, LA, Chicago, Houston, Miami and so on. The smaller cities and towns would be left out in the cold.
There may be some very good reason that I'm missing that argues against a Congressional District selection of Electors, but I can't think of it.
Not every CD would go for the Presidential candidate whose party is represented by the Member of Congress, so it is not a direct one-to-one relationship.
I understand it would make it much harder for the networks to "call" a state the moment the polls close based upon exit polls, but the method of electing a President shouldn't be designed for the convenience of the national press corps.
Over the next days or weeks researchers will have voting results that are granular to be able to determine what would have happened if the CD system were in place in 2012; but it wasn't so the two campaigns didn't design their efforts to reflect it.
The Constitution allows State Legislatures to determine the method of choosing Electors, so this system doesn't need a Constitutional amendment; it could be done in the next few months.