Sen. Hillary Clinton, D-N.Y., is back. By winning the Democratic primaries in Texas, Ohio and Rhode Island this week, Clinton has ensured the Democratic presidential nomination will be decided at the Democratic National Convention. She has also demonstrated that Sen. Barack Obama, D-Ill., is far from unbeatable.

Most of all, Clinton's comeback shows that both Clinton and Obama will have trouble handling Republican presidential nominee Sen. John McCain come November.

This year's primary cycle has been an exercise in political underdog-ism. Once a candidate becomes the odds-on favorite to win, expect him or her to lose.

In the Republican primaries, McCain was written off in mid-2007. Today, every Republican frontrunner -- Rudy Giuliani, Mitt Romney, Fred Thompson, Mike Huckabee -- is gone. McCain's the only man left standing. On the Republican side, flying below the radar was the best strategy.

On the Democratic side, the candidate rallying the most sympathy has carried the day. Early on, Clinton was the "inevitable" nominee. Obama's underdog status made him a sympathetic figure, the voice of truth fighting overwhelming odds. Over the past month, Obama's nomination assumed an air of inevitability -- and the sympathy swung to Hillary, the seasoned veteran, the political soldier fighting for every last hilltop. Obama, meanwhile, was subjected to the scrutiny every frontrunner faces -- and he was found wanting.

In the general election, any Democratic nominee is widely perceived as the favorite to take the White House. This leaves the Democrats in the unenviable position of facing frontrunner scrutiny -- and losing the sympathy that naturally flows to the underdog.

Obama couldn't stand up to the mild media pressure brought to bear by Hillary's recent commercial targeting his experience, the Rezko scandal and his NAFTA waffling. That pressure is nothing compared to the firestorm that awaits him in a general election. As for sympathy -- that, too, will be in short supply for a candidate who has waltzed to electoral success without facing substantial hardship in either his personal or political life.

As for Hillary, her inevitability shattered on contact with an inexperienced one-term senator. What will happen when her newly restored aura of inevitability clashes with an unfriendly American public, rather than the gushing supporters she almost lost in the primaries? It's difficult for Clinton to play the victim when she's seen as the heavy rather than the pugnacious scrapper.

The Democrats' early advantage leaves John McCain precisely where he wants to be -- flying below the radar, garnering the warm feelings that will assuredly accrue to a man who has served his country for decades and now faces an uphill battle. McCain is eminently human, a quality that both anointed demigod Obama and ice queen Clinton lack. His age may actually help him here -- if the American public gets the feeling that McCain is defending himself from the bullying tactics of a younger frontrunner, it may rally around him.

In 2008, leading the charge has largely resembled leading Pickett's charge -- the first in the footrace is the first to fall. The underdog has the sympathy. The underdog can avoid the glaring, weakness-exposing political spotlight. Advantage: McCain.