Weighing in as President Obama's sparring partner in debate prep, Massachusetts Sen. John Kerry said, "I don't believe the American people can trust" Mitt Romney.

"Mitt Romney comes in tonight and thinks he can just 'etch a sketch' the American people," Kerry said overnight in a post-debate interview, slamming the former Massachusetts governor for changing his position on issues such as women's health.

"That's an insult to the intelligence of Americans and it's an insult to the presidency. The presidency is worth more than that. It's certainly worth the truth, and Mitt Romney just didn't tell it tonight."

Kerry pointed specifically to Romney's onetime support for legislation that would have limited access to contraceptive services and the math to explain Romney's current tax proposals.

"Mitt Romney can't add up his tax numbers," Kerry said. "His tax plan doesn't work. It's been thoroughly exposed."

And with scorn, Kerry, Massachusetts' senior senator and Democratic presidential nominee who lost to George W. Bush in 2004, said, "It is unheard of that a former governor running for president cannot even carry his own state."

Massachusetts is considered safe Obama territory on Election Day.