John McCain, right, wife Cindy, watch Blue Angels fly-over at Meridian Naval Base in Mississippi, 30 Mar 2008

Republican presidential candidate John McCain has embarked on a "Service to America Tour" to highlight his and his family's background of military service.

McCain spoke Monday, in Meridian, Mississippi, a city where a naval airfield is named for McCain's grandfather, who was an admiral. McCain's father also was an admiral.

McCain, a navy pilot and a prisoner of war during the Vietnam War, said his father and grandfather were his first heroes, and that their respect for him has been one of the most lasting ambitions of his life.

The Arizona senator has won enough delegates to clinch the Republican nomination.

For the Democrats, Senators Hillary Clinton and Barack Obama are campaigning in Pennsylvania, the next battleground state in the race for the party nomination. The state holds its primary April 22.

Supporters of Obama have urged Clinton to drop out so the party can unite behind Obama, an Illinois senator who is leading in states won, delegates committed and popular votes. Clinton has rejected those calls.

Sunday in Pennsylvania, Obama said his attitude is that Clinton can run as long as she wants. He described the New York senator as a "fierce and formidable competitor."

For McCain, other stops on his four-state, five-day tour include his high school in Arlington, Virginia, the U.S. Naval Academy in Annapolis, Maryland, which he graduated from, and the Florida air station where he trained as a naval aviator.

Some information for this report was provided by AP.