HOUSTON — Senator Obama accused Senator Clinton yesterday of trying to “play on people’s fears to scare up votes” with a television ad showing sleeping children and asking who would be more qualified to answer a national security emergency call at 3 a.m.

“The question is not about picking up the phone. The question is: What kind of judgment will you make when you answer?” Mr. Obama said as he campaigned in Texas ahead of crucial contests here and in Ohio on Tuesday.

VIDEO: The ‘Children’ Ad

“We’ve had a red phone moment. It was the decision to invade Iraq. And Senator Clinton gave the wrong answer. George Bush gave the wrong answer. John McCain gave the wrong answer,” Mr. Obama said.

Mr. Obama’s response to the ad — which began airing in Texas on Friday morning — was a swipe at both Mrs. Clinton and the likely Republican presidential nominee.

Polls show a tight race between Mr. Obama and Mrs. Clinton in both states. Military issues play well in Texas, home to 16 active-duty military bases, including Fort Hood, the nation’s largest Army post.

Mrs. Clinton is casting herself as the candidate with the experience and judgment to take command on Inauguration Day. Mr. Obama, a first term senator, is seeking to chip away at those arguments by suggesting he would have superior judgment.

To the sound of a ringing phone, the Clinton ad shows children sleeping at night and a mother checking on a child as an announcer says a phone is ringing in the White House and something has happened in the world. It ends with an image of Mrs. Clinton on the telephone as the announcer asks, “It’s 3 a.m. and your children are safely asleep. Who do you want answering the phone?”

Mr. Obama responded: “We’ve seen these ads before. They’re the kind that play on peoples’ fears to scare up votes. Well, it won’t work this time.”

In a reference to Senate votes by both Mrs. Clinton and Mr. McCain in October 2002 to authorize Mr. Bush to use force in Iraq, Mr. Obama said that he voiced opposition to that war vote when he was an Illinois state senator. He said it would cost thousands of lives and billions of dollars.

“I said that it would distract us from the real threat we face and that we should take the fight to Al Qaeda in Afghanistan. That’s the judgment I made on the most important foreign policy decision of our generation, and that’s the kind of judgment I’ll show when I answer that phone in the White House as president of the United States.”

“That’s the judgment we need at 3 a.m. And that’s the judgment that I am running for President to provide,” he added.

The Obama campaign also responded to the new Clinton spot by re-airing an ad in which the Air Force chief of staff between 1990 and 1994, General Merrill McPeak, endorses Mr. Obama.

Addressing 60 veterans and their families at a town hall meeting at American Legion Post 490 in Houston, Mr. Obama said, “Veterans are bearing the brunt of bad decision-making by our leaders.”

The president’s job is “to keep people safe. … It means deploying our military wisely,” he continued. “War should not be the first resort. … lt should not be based on politics.” He promised to improve health care and other services for veterans if elected. “We are not serving our troops and our veterans as well as they have been serving us,” Mr. Obama said.

Since the Democratic debate in Cleveland on Tuesday, Mr. Obama until had steered clear from mentioning Mrs. Clinton by name in his appearances, while repeatedly criticizing Mr. McCain over his support for Iraq and for Mr. Bush’s economic policies.

Mr. Obama did not serve in the military, but told his audience that he comes from a military heritage.

“My grandfather enlisted after Pearl Harbor and marched in Patton’s Army. My mother was born at Fort Leavenworth, and my grandmother worked on a bomber assembly line. After his service, America stood by my grandfather. He went to college on the GI Bill, and bought his first home with help from the Federal Housing Authority. Then he moved his family west to Hawaii, where I was born. Today, he is buried in the Punchbowl Cemetery, where 776 victims of Pearl Harbor are laid to rest,” Mr. Obama said.