Let’s get real. If John McCain had a long-standing professional relationship with someone who had bombed abortion clinics, Democrats would never let him hear the end of it. And properly so. We’re engaged in an American presidential election here, not a tea social.

Got a mad bomber in your past? Sorry, it’s an issue.

It’s therefore neither shocking nor surprising that Republicans are attempting to exploit Barack Obama’s relationship with William Ayers, the former Weather Underground extremist who bombed the Pentagon to protest the Vietnam War. This was not only absolutely predictable, this column foresaw it months ago.

Never mind that Obama was 8 years old when Ayers’ insane act took place in 1970; nor that their relationship doesn’t appear to have been close. They did serve together on charitable boards in Chicago, where the ostensibly rehabilitated but unrepentant bomber is a professor of education at the University of Illinois. They appeared together in panel discussions; Obama wrote favorably about Ayers’ 1993 book “Fugitive Days: A Memoir,” in the Chicago Tribune; in 1995, the up-and-coming politician attended a reception in Ayers’ Hyde Park home.

That’s about the size of it. There’s no evidence Obama approves of Ayers’ violent past. None. Why he chose to dissemble when ABC’s George Stephanopoulos asked him about it, only Obama knows. He incorrectly described the former radical as an “English professor” and slight acquaintance who lived in his neighborhood.

Like his repudiation of the Rev. Jeremiah Wright, whose impolitic sermons (“God damn America!”) Obama implausibly pretended never to have heard, the candidate’s evasiveness was disingenuous at best.

So the Democratic nominee definitely had it coming. But it’s beginning to look as if he was politically shrewd. See, the preposterous Gov. Sarah Palin is running around accusing Obama of “palling around with terrorists.” She tells audiences of True Believers that Obama “is not a man who sees America the way you and I see America.”

According to Time magazine, the chairman of the Republican Party in Virginia, one Jeffrey M. Frederick, explicitly compared the Democratic presidential nominee to Osama bin Laden: “Both have friends that bombed the Pentagon,” he said. “That is scary.”

McCain artlessly tries to have it both ways. He chastised one elderly nitwit who called Obama an “Arab,” doubtless synonymous with “terrorist” in her mind. He called the Democratic nominee “a decent family man with whom I have some disagreements.” But he also declined to criticize Frederick.

“Sen. Obama ought to be candid and truthful,” McCain said, “about his relationship with Mr. Ayers in whose living room Sen. Obama launched his campaign and Sen. Obama said he was just a guy in the neighborhood.”

Hotheads at some of Palin’s rallies have yelled out “terrorist” and “treason” at the mention of Obama’s name. “Kill him!” one crackpot reportedly hollered, as the onetime Miss Congeniality chattered blithely on. Possibly she has a hearing problem, because an American politician with the intellectual acumen God gave a squirrel would know better than to leave the impression she finds such remarks acceptable.

This has, in turn, alarmed commentators sympathetic to Democrats. New York Times columnist Frank Rich accuses Republicans of stoking Nazi-style rage that could encourage assassins. “The McCain campaign,” he writes, “has crossed the line between tough negative campaigning and inciting vigilantism, and each day the mob howls louder.” Bloggers on the Chicken Little Left express fear of what one called “rubes with pitchforks out in Jesusland.”

On ABC News’ “This Week,” Nobel laureate Paul Krugman (has any American public intellectual ever deserved the honor more?) supplied much-needed historical perspective: “For a long time, we have had a substantial fraction of the Republican base that just does not regard the idea of Democrats governing as legitimate. Remember the Clinton years. It was craziness, right? They were murderers, they were drug smugglers, and the imminent prospect of what looks like a big Democratic victory would drive a lot of these people crazy even if Sarah Palin wasn’t saying these inflammatory things.”

Except that during the Clinton years, much of the “mainstream” press collaborated in peddling Looney Tunes story lines.

The ongoing catastrophe of the Bush administration, however, appears to have helped rationally consequent minds to sober up. Polls show voters taking the November election with unusual seriousness. Everybody knows somebody who went nuts over Vietnam. People want substance this time.

ABC News reports that Americans find McCain/Palin more focused on personal attacks than discussing issues by 59 percent to 35 percent. (Among independents, it’s 68 percent to 26 percent.) A Fox News poll (Fox, mind you!) found Americans saying the “Obama-Ayers connection” wouldn’t cause them to vote against the Democrat by 61 percent to 32 percent.

In other words, the ugly tone of McCain/Palin rallies doesn’t demonstrate growing intolerance and hatred. What it shows is that hardly anyone but far-right soreheads is showing up at GOP rallies anymore.