Fact-checking the statements of politicians, a growing trend in recent years, has been especially visible in this year's presidential campaign, thanks to the likes of Politifact.com, Factcheck.org and fact-checkers in the mainstream media as well.

Democrats have latched on to fact-checkers' reports labeling Mitt Romney and Paul Ryan serial liars. Republicans have claimed that the supposedly independent fact-checkers are liberal journalists in bed with the Democrats. Is such fact-checking a legitimate enterprise, or an attempt to hide bias behind a neutral façade? Who will fact-check the fact-checkers?

Take one widely ridiculed Ryan "lie": the claim that President Obama broke a campaign promise to keep a Janesville, Wis., auto plant open if elected. Several fact-checking reports labeled the charge false because it implied that the plant had closed on Obama's watch; in fact, it ceased production in December 2008 -- before Obama took office.

Not so fast, said conservative bloggers. Some operations at the plant continued well into 2009. Moreover, the plant is still "on standby" and could be reopened (though its prospects dimmed when General Motors decided to reopen another idled plant, in Tennessee).

The Janesville plant was already slated for shutdown in February 2008 when then-candidate Obama chose it as the location for a major speech and said, "I believe that if our government is there to support you ... this plant will be here for another hundred years." A few months later he reiterated that, as president, he would "lead an effort to retool plants like the GM facility in Janesville."

Of course, Obama made no guarantee to keep the plant open. But Ryan's basic point -- that Obama made a lot of grand promises on which he did not deliver -- stands.

For a much more complex fact-checking debate, there's the question of whether the Obama administration has, as Romney charges, sought to "gut" the work requirement for welfare recipients that was central to President Bill Clinton's 1996 welfare reform.

Democrats dismiss this claim as "thoroughly debunked": according to fact-checkers at Politifact, CNN.com and other venues, the Obama administration has merely agreed to grant waivers at the request of states -- some led by Republican governors -- and only if the states can demonstrate that their proposals can ultimately move more people into jobs.

But some experts -- not only Robert Rector of the conservative Heritage Foundation, but also maverick Democrat Mickey Kaus -- have ventured to rebut the rebuttal. They argue that the waivers come with such flexible standards they could make the work requirement meaningless. Even Ron Haskins, an analyst with the Brookings Institution, who supports the waivers, thinks they represent a major enough change to the 1996 law to need congressional approval.

Especially on complex policy issues, facts are rarely just facts. Is Obama's health care law a "government takeover" of health care, or merely an expansion of government's role? Would Ryan's Medicare reform plan represent the "end" of Medicare, or merely an overhaul? Is rhetorical exaggeration a lie? Is an out-of-context statement false?

Not all questionable fact-checks are slanted left. Factcheck.org brands as a falsehood the Obama camp's claim that the Republican Party platform would ban abortion even in cases of rape and incest; the platform simply doesn't mention these exemptions. Yet, since it states that "the unborn child has a fundamental individual right to life," is it illogical to conclude that this right would apply to unborn children conceived through rape?

Liberal bias or no, it's always a slippery slope from fact-checking to fact interpretation and labeling dissenting analysis a lie. What should pundits do? Either offer clearly labeled opinions, or present the facts as thoroughly as they can and leave readers to draw conclusions.

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Cathy Young, a regular contributor to Reason magazine and the website RealClearPolitics, wrote this article for Newsday.