With a little more than one week left in the U.S. presidential primary season, Democratic candidate Hillary Clinton has again vowed to continue her campaign because a large number of voters still have not cast their ballots.

Senator Clinton campaigned Sunday in the U.S. territory of Puerto Rico, which holds its primary next Sunday, June 1. She told a church congregation that faith has helped her through the campaign.

South Dakota and Montana vote two days after Puerto Rico.

Clinton is trailing fellow Senator Barack Obama in the number of delegates needed to secure the party's nomination.

On Sunday, Obama delivered the commencement address at Wesleyan University in the northeastern state of Connecticut. He was filling in for Democratic Senator Edward Kennedy, who was recently diagnosed with a cancerous brain tumor.

Obama urged the graduates to follow Kennedy's example of public service.

Republican Party candidate John McCain Sunday hosted at least three potential running mates at his home in Sedona, Arizona.

Florida Governor Charlie Crist, Louisiana Governor Bobby Jindal and former Massachusetts Governor Mitt Romney met with McCain at his ranch.

A third, much smaller U.S. political party, the Libertarian Party, on Sunday selected former U.S. Republican Congressman Bob Barr to be its candidate in the November presidential election.

Barr left the Republican Party in 2006 over what he called bloated spending and civil liberties intrusions by the Bush administration.