(Jason Reed/REUTERS)

Eight years ago, when Sen. John Kerry (Mass.) was the Democratic nominee for president, he blasted then-President George W. Bush for his record on jobs. He often noted that Bush was on track to be the first president since Herbert Hoover to preside over a net loss in jobs.

Bush, by contrast, repeatedly proclaimed that 1.7 million jobs had been added since April 2003 — a cherry-picked date designed to put his jobs record in the best possible light.

With the economic shoe on the other foot, now it’s GOP presidential nominee Mitt Romney who uses the attack lines honed by Kerry, and it’s President Obama who relies on Bush’s strained economic accounting.

Such manipulation of statistics is standard fare for political campaigns, which is one reason why voters should always take such claims with a grain of salt. Yet something deeper and more disturbing is also affecting the political system.

With the conclusions of the back-to-back political conventions, it is clear that both parties occupy parallel universes, with their own narratives and starkly different philosophies. The Fact Checker column tries to steer clear of philosophical disputes, but increasingly claims made by politicians are geared for the echo chamber of the party faithful.

As Democrats gathered in Charlotte for their convention, for instance, some readers raised questions about the opening line of a brief history of the political party on its Web site: “For more than 200 years, our party has led the fight for civil rights, health care, Social Security, workers’ rights, and women’s rights.”

The sentence, and the accompanying history, seemed determined to airbrush out of existence the Democrats’ troubled history on race relations, which lasted until the signing of the 1964 Civil Rights Act by President Lyndon Johnson firmly turned the bulk of the party in a new direction. (Still, 80 percent of the “no” votes in the Senate came from Democrats; the legislation would not have passed without the decisive support of Republicans.)

Meanwhile, Rep. James E. Clyburn (D-S.C.), in a speech to the convention on Thursday evening, gave Democrats full credit for the passage of Social Security in 1935 and Medicare in 1965, declaring that Republicans “just cursed the darkness” and “stood on the sidelines” when that legislation emerged from Congress.

In fact, Social Security passed with significant Republican support, and Medicare was adopted with “yes” votes by a majority of Republicans in the House and a significant minority in the Senate.

(Clyburn uttered those phrases even after his revisionist history was criticized by FactCheck.org and had received Four Pinocchios from this column, based on an advance text distributed by Democrats.)

The bipartisan majorities of those past achievements are much less likely now. Back then, voting broke more along regional divisions than party lines, with southern Democrats in opposition and northern Republicans in support of such federal expansion of benefits. Many of those southern Democrats have since become Republicans, while the Northeast, especially, has turned largely Democrat.

The parties have become so homogenized, with so little internal disagreement about the challenges they believe the country faces, that the portraits they painted of the United States at the conventions represented two starkly different visions.

From the Democratic side, the focus was on creating a level playing field for Americans who face obstacles or are disadvantaged, with the main tool usually government action.

Here is how Massachusetts Senate hopeful Elizabeth Warren described the country in a speech to the convention: “The system is rigged. Look around. Oil companies guzzle down the billions in profits. Billionaires pay a lower tax rate than their secretaries, and Wall Street CEOs, the same ones that direct our economy and destroyed millions of jobs, still strut around Congress, no shame, demanding favors, and acting like we should thank them.”

And, for Republicans, although the concern was also on the individual, it centered on reining in a government that they believe has run amok.

This is how former Arkansas governor Mike Huckabee portrayed the nation in his speech at the GOP conclave: “We [Huckabee and Mitt Romney] are mutual opponents of the miserably failed experiments that have put this country in a downward spiral. The United States of America was originally an experiment. But it was an experiment in recognizing God-given individual liberty and creating a government in which we no one is deemed better than another. And in which all of us are equal.”

This poses a fact-checking conundrum. One can question some of the specific claims, such as billionaires paying a lower tax rate than their secretaries. (The data show that is not especially common, just as the data suggest that voter fraud, a major concern of Republicans, happens only on rare occasions.) But it is virtually impossible — and probably foolhardy — to fact-check such deeply held beliefs.

In the halls of Congress, gerrymandered districts have reinforced these trends. Increasingly, the most important races are the primaries, the preserve of the party faithful, not the general election votes. That means members are more likely to represent the left and right wings of the parties, with fewer and fewer centrists becoming lawmakers.

If the two sides cannot even agree on the basic facts, then it becomes difficult to understand how they ever could reach agreement on the key issues that face the nation — such as strengthening the finances of the entitlement programs created many decades ago with bipartisan support that would be unimaginable today.

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