McCain's sharply worded criticism was not limited to the ad. He said yesterday that Obama would be facing a far less secure Iraq "if we had done what he wanted to do."

The 30-second ad, running on national cable and in 11 battleground states, is the hardest hit aimed at Obama so far by McCain.

"Barack Obama never held a single Senate hearing on Afghanistan," the ad's announcer says. "He hasn't been to Iraq in years. He voted against funding our troops. Positions that helped him win his nomination. Now Obama is changing to help himself become president."

The ad suggests that Obama was placing politics ahead of the country's interests. "John McCain has always supported our troops and the surge that's working," the ad states. "McCain. Country first."

McCain could have let the Republican National Committee air the commercial, as it did with another critical portrayal of Obama. But McCain aides say Iraq and national security are issues that McCain is eager to debate with Obama and saw no reason to put distance between himself and the criticism.

The ad is airing as Obama launches a foreign trip designed to ease voter concerns about his national security credentials. It also airs as Obama's commitment to withdrawing combat troops within 16 months of becoming president has come under question.

Obama has said he remains steadfast in his goal to end the war. While he had earlier said he would "immediately" begin to remove troops, he now says he would immediately give his commanders a mission to end the war. He also has said he remains committed to a withdrawal over 16 months.

McCain has been a staunch proponent of a military escalation that has increased security in Iraq. Obama opposed that surge in troops.

The ad's reference to Obama not holding hearings on Afghanistan refers to Obama's status as chairman of the Senate's subcommittee on European affairs. The panel's jurisdiction includes NATO forces fighting in Afghanistan.

Obama was last in Iraq in 2006; McCain was there in March of this year. Only once has Obama voted against money for troops in Iraq and Afghanistan - in May 2007. The bill passed overwhelmingly anyway. Otherwise, he has voted for every bill that financed the troops since he has been in the Senate.

Obama spokesman Bill Burton said yesterday that "while Barack Obama wants to change American foreign policy to wind down the war in Iraq and address the grave threat posed by a resurgent al-Qaeda and Taliban in Afghanistan, John McCain offers this patently misleading negative ad."

"Given his calls for a civil campaign," Burton said, "it's disappointing that Sen. McCain has slipped so easily into the same, tired campaign tactics that have become so familiar to the American people."

The commercial is McCain's first negative television ad against Obama, although the RNC has aired ads critical of Obama's stance on energy.