The 2012 presidential primary season officially ended as Mitt Romney won Utah's primary on Tuesday, adding 40 delegates to his credit.
Romney officially clinched the GOP nomination in May, but his candidacy against Obama became a foregone conclusion when Rick Santorum dropped his White House bid in April.
Some states might still adjust their delegate lists for the Republican National Convention in Tampa.
However, according to the Associated Press tally Romney currently has 1,512 delegates. He only needed 1,144 delegates to clinch the nomination.
There are primaries still to come for Congress.
Meanwhile, US President Barack Obama is ramping up a fresh phase of his re-election bid with a bus tour next week.
The bus trip also will coincide with the release of the monthly nationwide jobs report, a key economic indicator that could directly affect the president's re-election prospects.
The most recent report, for May, showed a slight uptick in the unemployment rate, raising fears among Obama aides of an election-year economic slowdown.
American voters are deeply divided on whether Romney or Obama is best suited to invigorate America's stagnant economy, which analysts say is this election's leading issue.
Obama's schedule – thus far focused on ritzy fund-raisers – will start to include campaign rallies and other events focused on speaking directly to American voters in the states Obama needs most if he is to hold the White House.
Ohio and Pennsylvania, with 38 electoral votes between them, are crucial to the president's re-election bid. Obama won both states in 2008, but Republican rival Mitt Romney is expected to make a strong play for each.
Just over four months from Election Day, Obama aides consider Ohio a toss-up, but believe Pennsylvania is leaning in the president's favor.
Romney took a bus tour of his own through Ohio and Pennsylvania earlier this month in hopes of overcoming an already slim Obama lead in the polls.
Obama's itinerary for the bus trip was still being finalized, but an official who spoke with the Associated Press on condition of anonymity said the president probably would hold events in northern Ohio and western Pennsylvania.