Romney got another boost in the wake of his victory in Illinois when he won the endorsement of former Florida governor Jeb Bush. Bush issued a written statement that said it was time Republicans unite behind Romney as their nominee.

In his speeches, Romney mentions Santorum less and less and focuses most of his attention on drawing a contrast with President Obama.

"The genius of America is that we nurture those dreams and the dreamers," said Romney. "We honor them and yes, we reward them. That is part of what is uniquely brilliant about America. But day by day, job-killing regulation by job-killing regulation, bureaucrat by bureaucrat, this president is crushing the dream and the dreamers and I will make sure that finally ends!"

Romney's base of support among Republican voters includes those who say their top priority is defeating Obama in November. He also does well among Republicans who describe themselves as moderates.

Rick Santorum continues to draw support from religious voters and from Republicans who consider themselves very conservative. He argues that he would offer general election voters a bolder alternative to the president than would Romney.

"They want someone who is not going to go to Washington, D.C. because they want to be the most powerful person in the world to manage Washington," said Santorum. "They want someone who is going to take that power and give it back to the people of this country!"

It takes the support of 1,144 delegates to secure the Republican nomination at the party's nominating convention in Tampa, Florida, in late August.

The latest Associated Press unofficial tally gives Romney more than 560 delegates, more than twice as many as Santorum.

Santorum is favored in the next primary in Louisiana on Saturday, but expert Henry Olsen of the American Enterprise Institute says Romney remains the strong favorite to eventually win the nomination.

"I think right now the way things are going, Mitt Romney just needs to be patient and he's going to be the nominee," noted Olsen.

Some analysts note that Republican voter turnout in the primaries and caucuses this year is down from four years ago. Some party leaders have also expressed concern that the lengthy and divisive primary campaign could hurt Republican chances in November.

But John Fortier of the Bipartisan Policy Center expects that the Republican Party will unify behind its nominee in time for the general election campaign in November.

"I think there is a great likelihood at the end of the race that most Republican voters will come home, just as after the [Hillary] Clinton and [Barack] Obama race where there was bitterness and different types of voters, most Democratic voters ended up supporting the nominee," Fortier explained.

The delegate math demonstrates how difficult it would be for Santorum to overtake Romney at this point. Santorum would have to win about 70 percent of the remaining delegates at stake to win the nomination, while Romney only needs to win about 45 percent to emerge victorious.