Republican John McCain is most militantly pro-free trade presidential candidate. That fact, alone, should guarantee his defeat in Ohio and other industrial states where his strategists entertain hopes of surfing a “Reagan Democrat” crossover of working-class Democratic voters to the GOP column this fall.

All that is required is that Barack Obama campaign as a critic of the North American Free Trade Agreement and other deals that have battered workers, farmers, communities and the environment in the US and abroad.

Unfortunately, Democrat Barack Obama, who sent so many smart signals on trade issues when he was competing with Hillary Clinton for his party’s presidential nomination, appears to now be backtracking toward the insider territory occupied by McCain.

Obama’s interview with Fortune magazine — headlined “Obama: NAFTA Not So Bad After All” — is the best news the McCain camp has received since Mike Huckabee folded his run for the Republican nomination.

If Obama takes the economic issue that white working-class voters best understand off the table, he creates a huge opening for McCain in states such as Ohio, Pennsylvania, Michigan and Wisconsin.

And that is precisely what the Democrat cynically dismisses his appropriately anti-NAFTA rhetoric during the primary season as “overheated and amplified.”

In her interview with the candidate, Fortune‘s Nina Easton reminded Obama that earlier this year he had called NAFTA “devastating” and “a big mistake” and suggested that he would use an opt-out clause in the trade agreement between the US, Canada and Mexico to demand changes that would be more favorable to workers and farmers in all three countries.

Obama replied that, “Sometimes during campaigns the rhetoric gets overheated and amplified” — which would have been enough of an indication that he was backing off the stance that contributed significantly to his success in the February 19 Wisconsin primary that proved to be a critical turning point for his campaign.

But the presumptive Democratic nominee for president dug the hole deeper.

“Politicians are always guilty of that, and I don’t exempt myself,” he continued, suggesting that those who doubted his sincerity when he denounced NAFTA in a speech to Janesville, Wisconsin, autoworkers might have been right.

Abandoning the tough talk of the winter and spring, Obama sounded an awfully lot like free-trader McCain when he said he was for “opening up a dialogue” with trading partners Canada and Mexico “and figuring to how we can make this work for all people.”

Easton took it that way.

“The general campaign is on, independent voters up for grabs, and Barack Obama is toning down his populist rhetoric – at least when it comes to free trade,” she began. “In an interview with Fortune to be featured in the magazine’s upcoming issue, the presumptive Democratic nominee suggests he doesn’t want to unilaterally blow up NAFTA after all.”

Referring to Obama’s soft-peddling of the fair-trade position he embraced in the primary campaign, Easton writes, “That tone stands in marked contrast to his primary campaign’s anti-NAFTA fusillades. The pact creating a North American free-trade zone was President Bill Clinton’s signature accomplishment; but NAFTA is also the bugaboo of union leaders, grassroots activists and Midwesterners who blame free trade for the factory closings they see in their hometowns.

“The Democratic candidates fought hard to win over those factions of their party, with Obama generally following Hillary Clinton’s lead in setting a protectionist tone. In February, as the campaign moved into the Rust Belt, both candidates vowed to invoke a six-month opt-out clause (‘as a hammer,’ in Obama’s words) to pressure Canada and Mexico to make concessions… Now, however, Obama says he doesn’t believe in unilaterally reopening NAFTA.”

As David Sirota, the author of a terrific new book on populist anger at Washington’s trade and economic policies, The Uprising, correctly observes, “Here you have a policy — NAFTA — that is among the most unpopular policies of the last generation, according to polls. Here you have a candidate who campaigned against it in the primary. And within weeks of getting the general election, here you have that same candidate running to Corporate America’s magazine of record to reassure Wall Street about that same policy. This is precisely what the populist uprising that I describe in my new book is all about — a backlash to this kind of politics.”

The McCain camp is already suggesting his Democratic rival is hypocritical, at best, when it comes to trade policy. The Fortune interview will add fuel to the fire.

If Obama does not change his tune, he could get burned in Ohio, Wisconsin and other states where primary surveys showed that the vast majority of Democratic, Republican and independent voters felt that the radically pro-corporate free trade policies of the Clinton and Bush years had harmed rather than helped America.