"We don't want a president who invites testing from the world at a time when our economy is in crisis and Americans are already fighting in two wars," McCain, a 72-year-old Vietnam War veteran, told a crowd Monday in Belton, Mo.
"What is more troubling is that Sen. Biden told their campaign donors that when that crisis hits, they would have to stand with them, because it wouldn't be apparent Sen. Obama would have the right response," added the Republican nominee, who was spending Tuesday in Pennsylvania, another battleground. "Forget apparent. Sen. Obama won't have the right response, and we know that because we've seen the wrong response from him over and over during this campaign."
At weekend fundraisers, Biden said of Obama, "Watch, we're gonna have an international crisis, a generated crisis, to test the mettle of this guy."
Biden, however, drew a far different conclusion than McCain. He compared Obama to President John Kennedy and said, "They're going to find out this guy's got steel in his spine."
On Tuesday, McCain told voters in Pennsylvania about his personal connection to the Cuban Missile Crisis that Biden was referring to.
McCain recalled being ready to launch a bombing run during the October 1962 Cuban Missile Crisis.
"I was on board the USS Enterprise," McCain, a former naval aviator, said in the capital city of Harrisburg. "I sat in the cockpit, on the flight deck of the USS Enterprise, off of Cuba. I had a target. My friends, you know how close we came to a nuclear war."
As the crowd of several thousand began to swell with cheers and applause, he added with dramatic effect: "America will not have a president who needs to be tested. I've been tested, my friends."
McCain also is criticizing Obama's opposition to President Bush's decision to send tens of thousands of additional U.S. troops to Iraq, as well as his rival's more restrained response to Russia's invasion of Georgia this summer.
In an exclusive interview on CBS' The Early Show, McCain defended his attacks on Obama and addressed the perception that is may be costing him supporters.
"I think that whatever is on peoples' minds is valid, but, the thing that's intriguing about it, Senator Obama has spent more money on negative attack ads than any political presidential campaign in history," McCain told Harry Smith.
In the interview, McCain also dismissed the idea that he can't win the presidency if the top issue is the flagging economy. McCain said "it's absolutely not true" that the economy is a losing issue for Republicans. Earlier this month, the New York Daily News reported that a top McCain strategist said in an interview, "If we keep talking about the economic crisis, we're going to lose."
"We're focusing on the economy," the Arizona senator said. "Listen to me. I'm the candidate, and this campaign is about the economy."
Obama gained a forceful rebuttal to those concerns over the weekend, when former Secretary of State Colin Powell, a retired four-star general and former chairman of the Joint Chiefs of Staff, endorsed Obama and attested to his readiness to be president.
Powell also criticized McCain's selection of Alaska Gov. Sarah Palin as his running mate, saying she failed to meet the primary qualification for a vice president: the ability to assume the presidency at any time.
The attacks on Obama are one element in a sharpened stump speech in which McCain also accused his rival of having socialistic tax policies. They come two weeks before Election Day, and as Obama maintains a lead in national polling as well as in surveys conducted in key battleground states.
It was unclear whether McCain might step back from his attacks after Obama's campaign announced that he will suspend campaigning for two days later this week to visit his gravely ill, 85-year-old grandmother in Hawaii.
Amid concern that battleground states were slipping from their grasp, McCain aides scheduled a daylong tour across Pennsylvania on Tuesday.