“I kind of like today, this was a good day,” Romney said.

Santorum's departure came two weeks before primary voting in his home state of Pennsylvania, a contest political analyst Thomas Mann says Santorum was likely to lose.

“Realizing he was going to get trounced [defeated] for the second time in his own state - last time when he was defeated for reelection to the Senate - he decided it is best to get out before making more enemies in the Republican Party,” Mann said.

Social conservatives propelled Santorum to victory in 10 states, prolonging a bruising and costly Republican primary fight. For months, Santorum blasted former Massachusetts Governor Romney as a politically rudderless moderate.

“He is the worst Republican in the country to put up against Barack Obama,” Santorum said.

And, for months, Romney insisted he is a true conservative, staking out right-wing positions that could prove a liability in the general election. 

Now, with his nomination all but assured, Romney can focus attention and resources on President Barack Obama, who has been working to energize Democratic voters and appeal to independents.

“No matter where you come from, what you look like, what your last name is - black, white, Hispanic, Asian, Native American, gay, straight [heterosexual], disabled or not, it does not matter. You have a shot at the American Dream. That is the vision we share,” Obama said.

Already, Romney is facing questions about his eventual vice presidential pick and running mate.

“I am here to announce today that I do not even have a list [of vice presidential picks],” Romney said.

Romney’s emergence in April as the all-but-confirmed nominee gives him time to heal divisions within the Republican Party and craft a message that will appeal to broader segments of the American electorate. Public-opinion pollster Peter Brown says Romney has work to do to erase the after-effects of a vicious nomination battle.

“The Republican candidates for president have been kicking each other for several months, and that kind of negative tone rubs off [is damaging],” Brown said.

There was no negativity from Romney after the Santorum announcement.

“We are going to work together to make sure we take back the White House, the Senate, and we keep the House [of Representatives]. We are going to get the job done,” Romney said.