The number 270 will come to dominate almost every waking moment for the Obama re-election high command in Chicago—as well as for their counterparts in the headquarters of the GOP nominee next year.

Two hundred seventy is the number of Electoral College votes needed to win the White House. Strategists on both sides will obsess on how to cobble together enough states to reach that total.

Since the 2008 election, 18 states have experienced a change in their number of electoral votes because of the decennial census. Some (mostly red ones) have gained electoral votes and some (mostly blue) have lost electoral votes. John McCain would have closed the gap by 14 electoral votes in 2008 if the contest had been run under the 2012 Electoral College distribution.

Most states are not in play. Mr. Obama will not win Utah and Wyoming, and the Republican nominee will not carry the District of Columbia or Rhode Island. But right now 14 states (with 172 electoral votes) are up for grabs.

Mr. Obama narrowly won three traditionally Republican states in 2008: Indiana, Virginia and North Carolina.

Democrats last carried the first two in 1964 and the third in 1976.

The president will be hard-pressed to win these states and their 39 electoral votes next year, especially Indiana and North Carolina. Democrats will have their convention in Charlotte in an attempt to hold the latter. But a 2009 study by political scientists Michael J. Berry and Kenneth Bickers (of the University of Colorado at Boulder and Denver, respectively) found "no evidence that hosting a national nominating convention has any discernible effect on the ultimate vote in that state."

Ohio, with 18 electoral votes, and Florida, with 29, both went Democratic in 2008 (they went Republican in 2004), but the swing in each was less than the national average. This indicates some weakness for Mr. Obama that has persisted: A recent Quinnipiac University poll in Florida shows the president losing to a generic, unnamed Republican by three points.

Karl Rove is a Fox News contributor and former senior adviser and deputy chief of staff to President George W. Bush. He is a Fox News contributor and author of "Courage and Consequence" (Threshold Editions, 2010). To continue reading his column in The Wall Street Journal on the 2012 presidential campaign, click here.