Barack Obama said on Monday his own carelessness caused him to receive intense criticism when he did not place his hand over his heart while singing the Star Spangled Banner in Iowa last year and for not wearing the label flag pin.

Obama raised the matter today at an appearance in Independence, Missouri. It's the start of a weeklong focus on American values ahead of Friday's July 4th holiday.

At certain times over the last 16 months, 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 and doubts about who I am and what I stand for, he said.

Obama has been defending his patriotism ever since the beginning of the primary season. The presidential candidate came under fire as early as last September for not holding his hand over his heart while singing the Star Spangled Banner at the Harkin Steak Fry in Iowa.

Obama was also criticized for not wearing a flag pin, which he now does much more frequently, and when false rumors began circulating that he did not say the Pledge of Allegiance.

He also vowed to never question the patriotism of others in the campaign. Patriotism must involve the willingness to sacrifice, Obama said, adding that John McCain need offer no further proof of that sacrifice after enduring five years as a prisoner of war.

Meanwhile, over the weekend, retired General Wesley Clark - an Obama supporter - aroused controversy when he said that McCain's Vietnam War-era service does not necessarily qualify him to be commander in chief.

I don't think getting in a fighter plane and getting shot down is a qualification to become president, said Clark, who is a military adviser for Obama.