"I think it's important to note that Senator Clinton was a cheerleader for NAFTA for more than a decade," Obama said at a news conference with representatives of a regional United Auto Workers chapter, whose members voted last week to recommend an endorsement of the Illinois senator.
"I realize that changing your position to suit the politics of the moment might be smart campaign tactics but isn't the kind of strong, principled leadership America needs right now," Obama said.
The Illinois senator commented at the start of a two-day swing through Iowa, where the voting begins Jan. 3. He addressed several hundred attendees at town meetings in northeast Iowa and attended a fundraiser for Iowa Rep. Bruce Braley, a Democrat in his first term.
At a town meeting in Grundy Center, Obama engaged in a tense back-and-forth with Jane Svoboda, a local farmer who asked what he would do about terrorism and illegal immigration. Obama bristled at the notion that Republicans would be better protectors of national security.
"I have a 9-year-old daughter and a 6-year-old daughter. Don't think I care any less than Rudy Giuliani or Mitt Romney about making sure my daughters don't get blown up," Obama said. "I live in Chicago. It's much more likely than Grundy County for the next terrorist attack. So I take this deathly serious."
Polls show Obama, Clinton andin a tight race in Iowa even as Clinton holds a wide lead in most national surveys. All three candidates are campaigning in the state in the run-up to the Thanksgiving holiday.
Obama, whose largest base of support lies with affluent and well-educated voters, has sought to burnish his credentials among middle- and working-class Democrats. Polls show Clinton running strong among those voters, while Edwards has run a tough-talking populist campaign aimed at wooing that group as well.
The North American Free Trade Agreement was passed in 1993 with the strong support of President Clinton, and eliminated most tariffs on goods traded between the United States, Canada and Mexico.
Since then, many Democrats and representatives of organized labor have roundly condemned NAFTA, arguing that it has favored the rich while moving thousands of middle-class jobs out of the United States. Edwards has called the agreement a "disaster."
When pressed at a televised debate Thursday on whether she still supported NAFTA, Clinton said the agreement had been a mistake "to the extent that it did not deliver on what we had hoped it would."
Obama took note of those words Sunday.
"I was pleased to hear her say that because as more than 10,000 jobless Iowans know, that's what NAFTA has been," he said.
Obama also denounced as "slime politics" a flap created by a column published Saturday by conservative journalist Robert Novak.
In the syndicated column, in which he provided no sourcing or attribution, Novak wrote that Clinton's campaign had decided against spreading "scandalous" information about Obama - who challenged the rival campaign to either produce the information or refute it.
A Clinton spokesman said the campaign had no such information.
Obama said he took the Clinton team at its word, but insisted Sunday that he would never tolerate such tactics.
"I have lived for 46 years now, and have been in politics for close to two decades. I really value my reputation, my character and my family," he said. "If you don't get on this stuff quick it starts drifting around. That's not something I'm going to accept because that's exactly the kind of politics we need to change."
Obama compared the matter to 2004, when Democrat John Kerry's presidential campaign suffered severe damage amid a media campaign launched by a group calling itself the Swift Boat Veterans for Truth. The group challenged Kerry's service record in the Vietnam war, and the Massachusetts senator was widely criticized for failing to rebut their charges more quickly and forcefully.
"It must be the silly season, because this is getting ridiculous," Clinton campaign spokesman Phil Singer said when told of Obama's comments. "Senator Obama knows that our campaign responded instantly with a clear denial," Singer said.
Singer also rejected Obama's criticism of Clinton's record on NAFTA.
"The fact is that Senator Clinton has been voicing serious concerns about trade agreements for years and as a senator from New York, she didn't just talk, she acted," Singer said. He noted that she had had voted against the Central American Free Trade Agreement and had criticized the Bush administration for failing to enforce trade agreements. Clinton also supports a pending free trade deal with Peru.