"I understand that Sen. McCain was asked yesterday this question, 'How many houses do you own?,' and he couldn't answer that question. He couldn't count high enough apparently to even know how many houses he owns," Virginia Gov. Tim Kaine told CNN's "American Morning."
It's a tricky balance as Obama goes on the offensive against McCain but also tries to respect his White House rival's "compelling personal story."
Obama frequently mentions McCain's prisoner-of-war experience and today described it as something "none of us could imagine."
But the Democratic contender quickly pivoted to the attack.
"There's just a fundamental gap of understanding between John McCain's world and what people are going through every single day here in America," he said.
Before McCain's comments Wednesday, Democrats were more focused on whom Obama would select as his running mate. No doubt the choice will make a difference.
According to new polls from both the Wall Street Journal and The New York Times, the candidates are in a dead heat, with Obama polling at 45 percent and McCain polling at 42 percent -- both polls within the margins of error.
The figure that most concerns Democrats is the 44 percent of those polled who say Obama is prepared to be president versus the 68 percent who feel McCain is prepared.
The tightening poll numbers could have an effect on Obama's vice presidential pick.
"[The Obama campaign] certainly seems to be signaling that the person will have instantly recognizable national security experience that means Joe Biden, maybe Evan Bayh, not Tim Kaine of Virginia and maybe even the big surprise, you know, someone like Hillary Clinton or Al Gore," Stephanopoulos reported on "World News."
ABC News' Andrew Fies, Avery Miller and Natalie Gewargis contributed to this report.