Hoping to instill doubt about Barack Obama’s general-election viability, Clinton pollster Mark Penn raised an interesting point the other day.

“She has consistently shown an electoral resiliency in difficult situations that have made her a winner,” Mr. Penn said. “Senator Obama has in fact never had a serious Republican challenger.”

That’s certainly true. In 2000, he unsuccessfully ran in a tough primary against Rep. Bobby Rush, but Rush is a Democrat. In his various campaigns for the state Senate, Obama won fairly easily. In 2004, he ran in a tough multi-candidate Senate primary, but after securing the nomination, he ended up running against professional right-wing gadfly Alan Keyes. A “serious Republican challenger”? Hardly.

On the other hand, Hillary Clinton’s campaigns haven’t exactly been barn-burners. Clinton’s first-ever campaign, the Senate race in 2000, was supposed to be a serious campaign against Rudy Giuliani, but she ended up facing a feckless Rick Lazio, beating him easily. In 2006, with very strong poll numbers in New York, Republicans struggled to find anyone to run against her, ultimately settling on former Yonkers mayor John Spencer. Clinton never had to break a sweat on route to her landslide victory — Spencer finished with just 31% of the vote.

But here’s a fun little twist — guess who else has never really gone up against a serious challenger from the other party? John McCain.

November is bound to be interesting, isn’t it?