Democrat Barack Obama defended his own patriotism and vowed never to use the issue against anyone during the race for the White House, rejecting comments by a retired general and backer about the military record of his opponent, Republican John McCain.
Shortly after, the Obama campaign announced he and former President Bill Clinton had a "terrific conversation" by telephone, breaking an icy silence between two of America's most gifted Democratic politicians.
Obama has been attacked for not wearing an American flag lapel pin and for allegedly failing to place his hand over his heart during the pledge of allegiance to the flag.
He challenged his critics in a forceful speech in the aptly named city of Independence, Missouri, as he kicked off a campaign week that includes the July 4 US holiday celebrating independence from Britain.
Obama, baptized a Christian two decades ago, also has fought persistent Internet rumors that he is Muslim.
"I have found, for the first time, my patriotism challenged - at times as a result of my own carelessness, more often as a result of the desire by some to score political points and raise fears about who I am and what I stand for," he said before a crowd of a few hundred people at the Truman Memorial Building in President Harry Truman's hometown.
Obama, who has vowed to change the tenor of American politics by rejecting negative campaigning, also sought to distance himself from remarks retired Gen. Wesley Clark made regarding McCain during a weekend television interview.
Clark, who backed Hillary Rodham Clinton until she dropped out of the race for the Democratic nomination, said on CBS television on Sunday that McCain's military service and five-years as a prisoner of war in Vietnam did not necessarily qualify the four-term Arizona senator to be commander in chief. Clark, who sought the Democratic nomination four years ago, now backs Obama.
Clark's remarks set off a familiar pattern from earlier tussles over criticism: The candidates, Obama and McCain, took the high road while the harsh language was left to surrogates.
Obama, without mentioning Clark by name, said that patriotism "must, if it is to mean anything, involve the willingness to sacrifice."
"For those like John McCain who have endured physical torment in service to our country - no further proof of such sacrifice is necessary," Obama said. "And let me also add that no one should ever devalue that service, especially for the sake of a political campaign, and that goes for supporters on both sides."
McCain was asked about Clark's remarks during a news conference in Pennsylvania.
"I think that that kind of thing is unnecessary," McCain said. "I'm proud of my record of service, I have plenty of friends, leaders who will attest to that."
Separately, in a statement, Obama spokesman Bill Burton said, "As he's said many times before, Sen. Obama honours and respects Senator McCain's service, and of course he rejects yesterday's statement by Gen. Clark."
Obama, meanwhile, asked Bill Clinton to campaign for him when they had their first conversation since the heated Democratic primary contest was lost by the former president's wife.
Bill Clinton was often Obama's harshest Democratic critic, trying to upend the Illinois senator's candidacy in favor of his wife. While Hillary Clinton has begun to help Obama by encouraging her supporters to back him, a chill remained between the last Democratic president and the man running to be the next one.
Burton said they had a "terrific conversation" and that Obama is honoured to have the former president's support.
"He has always believed that Bill Clinton is one of this nation's great leaders and most brilliant minds, and looks forward to seeing him on the campaign trail and receiving his counsel in the months to come," Burton said.
Clinton spokesman Matt McKenna said the former president renewed his offer - expressed in a one-sentence statement last week - to do whatever he can to ensure Obama wins the presidency.
Obama spokesman Robert Gibbs said the senator called Clinton after Obama landed in Missouri on Monday morning, and they spoke for about 20 minutes. Gibbs said Obama asked Clinton to campaign with him and separately.
A new poll, meanwhile, showed 90 percent of Americans are feeling the economic squeeze of rising fuel prices, but many voters think neither Obama nor McCain can do much about it.
The Associated Press-Yahoo News poll released Monday underscored why Obama and McCain have sought to differentiate themselves on energy issues.
The survey highlights how energy costs have overtaken the Iraq war and terrorism as the dominant issue in their race.
With the holiday weekend approaching, the poll showed Americans were driving less, curtailing vacation plans and thinking about buying a car that uses less fuel.
While the AP-Yahoo poll showed a third of Americans felt both McCain and Obama would be unable to have much effect on fuel prices, those who did gave Obama a 28 percent to 20 percent edge over McCain, while 18 percent said they trusted the candidates equally.
McCain recently changed his position on the quarter-century US ban on drilling for oil and gas in offshore waters, saying states should be allowed to open those resources to oil companies if they wished.
He has also called for a major push to develop nuclear power and said he would give a $300 million prize to the developer of a battery capable of efficiently powering automobiles.
Obama has focused on increased spending for alternative energy sources funded in part by taxes on excess oil company profits and closing a legal loophole that he says has led to speculation on energy markets and driven prices to record levels.
Plans offered by both candidates were unlikely to have any immediate impact on soaring energy costs. The price of oil topped $143 a barrel for the first time Monday in electronic trading on the New York Mercantile Exchange. It closed at $140 a barrel, having shot up by 50 percent since the first of the year.
