Any doubt that Mitt Romney would win the Republican presidential nomination vanished when Rick Santorum left the race. It also marked the end of Romney’s time as the defining figure in the overall contest for the White House.

The GOP nomination process was seen by many as a competition between Romney and an entertaining cast of I’m Not Mitt Romney challengers. Questions were raised about Romney’s perceived weaknesses and whether he could win over the hearts and votes of conservatives. 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.