John Kerry says Republican John McCain doesn't have the judgment to be president.
If that's the case, then it's probably a good thing McCain rejected overtures from Kerry, the Democratic presidential nominee in 2004, to form a bipartisan ticket and run with Kerry as his candidate for vice president.
Kerry had no kind words his Senate colleague this weekend, accusing McCain of poor decision-making on everything from backing tax cuts for the wealthy to making support for continuing the U.S. military presence in Iraq the centre of his presidential campaign.
Kerry says McCain has proven that he has been wrong about every judgment he's made about the war.
He said McCain was wrong about the Iraqis paying for the reconstruction, wrong about whether the oil would pay for it and wrong about Sunni and Shia violence.
Kerry also said that McCain has been wrong about the willingness of the Iraqis to stand up for themselves.
Kerry supports Democratic presidential candidate Barack Obama.
"If you like the Bush tax cut and what it's done to our economy, making wealthier people wealthier and the average middle class struggle harder, then John McCain is going to give you a third term of George Bush and Karl Rove," the Massachusetts senator added.
That echoes an Obama campaign talking point.
Kerry later said the McCain of 2008 isn't the McCain he courted in 2004.
"John McCain has changed in profound and fundamental ways that I find personally really surprising, and frankly upsetting," said Kerry.
"It is not the John McCain as the senator who defined himself, quote, as a maverick, though questionable."
McCain adviser Senator Lindsey Graham of South Carolina said McCain was the one who stood up to the administration and urged President Bush to send more U.S. troops to Iraq to help control violence.
Obama did not support the addition of troops.
Violence in Iraq has dropped to its lowest level in more than four years as a result of the 2007 buildup of forces.
"We're winning because John McCain understood Iraq better than anybody else," Graham said. "The surge has worked. The political, economic and military progress in Iraq is undeniable."
Not too long ago, Kerry might have described McCain, a fellow Vietnam veteran and former prisoner of war, as a bipartisan ally who could provide guidance on national security issues.
McCain came to Kerry's aid in March 2004 after Bush and his campaign tried to paint the Democrat as weak on defence.
He rejected the suggestion in broadcast interviews and chided both parties for waging such a "bitter and partisan" campaign.
The two senators also discussed the vice presidency several times before McCain finally rejected Kerry's overtures to form a bipartisan ticket.
Kerry ultimately selected then-Senator John Edwards of North Carolina to join the ticket.
