"Every time they open their mouths they can't tell the truth," Kerry said at a rally. "It's time for us to have a president of the United States who can look you in the eye and when he does, you know you're being told the truth."
The newly combative senator from Massachusetts said he would not allow President Bush or others to question his fitness for the White House.
"Not gonna do it, not gonna happen," Kerry said. "I will not have my commitment to defending this country questioned by those who refused to serve when they had a chance and I will not have it questioned by those who misled this nation into war in Iraq."
Neither Bush nor Vice President Dick Cheney, who scathingly attacked Kerry's national security credentials at the Republican National Convention on Wednesday night, fought in Vietnam. Bush served in the Texas Air National Guard and Cheney obtained five draft deferments.
Kerry, a decorated Navy lieutenant who commanded gunboats in the Mekong Delta, has been criticized for failing to react quickly or strongly enough to advertisements challenging his service and his qualifications for the presidential role of commander-in-chief. On Friday, he assailed Bush for a "record of failure" on everything from jobs to Iraq.
He listed some of Bush's reasons for the U.S.-led invasion of Iraq -- weapons of mass destruction and connections to al Qaeda -- and added after each: "That wasn't true."
Kerry said Republicans had distorted his 20-year record in the U.S. Senate, including his vote authorizing the use of force against Saddam Hussein and his subsequent vote against $87 billion to fund operations in Iraq and Afghanistan. He predicted voters would reject cynical politics when they went to the polls on Nov. 2.
"All this negative advertising, blah blah blah," Kerry said. "It turns people off."
After ceding much of the political stage to the Republicans this week, Kerry hit the campaign trail with a two-day bus tour of Ohio, a critical battleground state hit especially hard by manufacturing job losses.
"There seems to be such a great gap, it's as big as the Grand Canyon, bigger, between the kind of things they talk about and the problems that are represented in people's lives," he said.
Kerry ridiculed the Bush administration's "celebration" of the August job numbers released on Friday that showed the unemployment rate had dropped from 5.5 percent to 5.4 percent with the creation of 144,000 new jobs.
"At the rate that this administration is creating jobs, you're not going to have a net plus-one job in the state of Ohio until the year 2011," he said. "I don't think this is something to celebrate. I think it's something to get to work on."
Kerry aggressively criticized the just-concluded Republican convention in New York that formally nominated Bush for re-election, although he said he "listened to zero of it."
"I read the speech last night of the president and I guess I'd sum it up in just four words: all hat, no cattle," he said.
"What we've learned now is that the president and the Republican Party will say anything and do anything in order to try to get re-elected -- anything except really take care of middle class American families that are struggling in this country."