I'm always amused when the Left trots out this tired, old canard. Clearly, if the Presidential election were a national popular vote referendum, campaign strategies would change. States that were clearly in one column would face increased pressure to turn out even more votes in a race already decided at the state level.
Imagine a World Series where the total number of runs over 7 games decided the winner. Game strategy would change immensely. And, not for the better.