But now President Obama moves to center stage and becomes the defining figure of the general election campaign. Now it's about Obama, not Romney, as the election becomes primarily a referendum on his first term in office.

The most important indicator of the president's prospects will be his job-approval rating. That rating will be very close to his share of the vote on Election Day. In 2004, President George W. Bush had a 51 percent job approval rating and won 51 percent of the vote.

Obama's ratings suggest we are heading for a potentially very close race in November. For the past 32 months, the full month approval ratings for the president have been remarkably stable, holding to a very narrow range of 44 percent to 49 percent. People seem to have formed an opinion of the president, love him or hate him, and nothing can change their minds. Those who oppose the president tend to feel more strongly about it than those who support him.

For most of the past three years, the president's ratings have stayed in an even narrower band of 46 percent to 48 percent. Those numbers suggest Obama would earn just under 50 percent of the vote on Election Day. If the president can win over a few more voters and move those numbers up a bit in the coming months, he is very likely to keep his job. If the president's ratings falter, Romney is likely to be moving into the White House next January.

Economic concerns dominate the voters' agenda, and here the numbers for the president are more troubling. Forty-nine percent of the nation's voters trust Romney more than the president when it comes to the economy. Just 39 percent trust Obama more.