"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." - Manuel II Palelologus
Unless congressional districts are drastically redrawn to favor the Democrats in just about every state, the GOP would have a *huge* advantage if every state adopted the congressional-district method of allocating presidential electoral votes (currently used only by Maine and Nebraska.
In 2004, President Bush carried 255 congressional districts (41 of which were won by congressional Democrats in 2004, and 20 more which were won by congressional Democrats in 2006) to only 180 for Kerry (18 of which were won by congressional Republicans in 2004, of which only 10 seats were still held by Republicans after 2006). Throw in the 2 extra EVs that would be given for carrying a state and the 3 EVs Kerry got from DC, and President Bush would have won reelection by 317-221 (instead of 286-252).
And since the states would not be winner-takes-all, it would have been impossible for Kerry to make up the difference by swinging one or two large states his way; that 96 EV margin would have constituted a landslide. In fact, President Bush would have received 278 electoral votes (8 more than needed to win) just in the states and districts he won with 53% or more of the vote, so even had there been a 6% swing towards Kerry it would not have been enough for Kerry to defeat President Bush.
That being said, the Constitution clearly provides that presidential electors are selected in such manner as provided by the respective state legislatures, so we won’t see a national system absent a constitutional amendment.