Simple. By allocating votes by Congressional District Mitt Romney would have been the winner. Romney won a majority of US Congressional Districts and if the vote were distributed by Congressional District, instead of Winner-Take-All Mitt Romney would have emerged victorious. In all states (with the exception of Nebraska and Maine) the winner of that state gets all of their electoral votes.
With this system in place Romney would have gained the majority of electoral votes from Pennsylvania, Ohio, Michigan, Virginia and Wisconsin. He would have lost a few electoral votes in Utah, Texas and Alabama.
From The Daily Mail: