As the presidential contenders from both parties start being placed by conventional wisdom into two camps, top tier and second tier, the Republican and Democratic front-runners seem most notable these days for what they all have in common: Each of them has at least one glaring flaw, and that presents plenty of opportunities for the others to move up from also-rans in the most wide-open race for the White House in eight decades.

It’s tough to tell if this is an unusually deficient crop of major contenders or whether the intense focus on their shortcomings is just a natural part of the early hustings in modern presidential campaigns. Either way, the foremost hopefuls are each battling grave enough concerns about their chances that you have to wonder if they are really up to the challenge of this extraordinarily momentous election.

Already, the 2008 campaign is historic because of its lack of an heir apparent; 1928 was the last time when neither the president nor vice president was in the hunt for the top job. Even President Harry S Truman was on New Hampshire’s Democratic primary ballot in 1952, despite already having decided not to run again. After finishing second to Sen. Estes Kefauver of Tennessee, he made his non-candidacy official and his vice president, Alben Barkley, briefly sought the nod before being rebuffed by party leaders.

With George W. Bush term-limited, Dick Cheney disavowing any interest in moving up, and the previous two Democratic nominees not in the hunt, this should be anybody’s race. And it may still be. To see how easily the top tiers could shake loose, look no further than how the leading candidates are being forced to launch their bids with tactics aimed at surmounting their liabilities.

On the Republican side, John McCain faces gripes that he’s lost his once-golden status as a maverick who shuns partisan politics and talks straight. That’s one reason, his advisers say, that the Arizona senator has so determinedly backed Bush’s plan to send more troops to Iraq. Such a strong stand against the flow of public sentiment is meant to demonstrate he’s still got the same sort of political courage that helped fuel his first presidential bid, in 2000.

Rudolph Giuliani is making an effort to put the best light on his personal life. His third wife, Judi, is doing interviews talking up the former New York mayor’s family values — a tactic aimed at dispelling concerns among social conservatives about his multiple marriages and the adultery charges against him in his 2002 divorce.

Mitt Romney bears the burden of his religion. The former governor of Massachusetts would be the first Mormon president, but polls show a substantial number of Americans won’t vote for a Mormon. Those numbers rise among evangelical Christians, a reliable voting bloc in GOP primaries. In a New York Times interview last week about his religion, Romney distanced himself from obsolete Mormon practices such as polygamy, which he described as “bizarre,” and he disputed the argument that his faith would require him to be loyal to his church before his country.

Hillary Rodham Clinton launched her bid for the Democratic nomination with a relentless effort to re-introduce herself and convince voters that, despite her fame, they don’t know the real person behind the image. Her pitch is aimed at dumping the political baggage, which got filled when she was first lady, that makes her such a divisive figure. But on her first trip to Iowa as a candidate, the New York senator got a telling reminder of how her husband could interfere. The national media obsessed on an oblique remark appearing to refer to former President Bill Clinton as a “bad man,” even though she denied any such intent.

Barack Obama does what he can to quell concerns that, after just two years in the Senate, his experience cannot match his charismatic presence on the presidential stage. Perhaps his best argument is when he notes that Abraham Lincoln, another tall man from Illinois, had even less experience before becoming president.

John Edwards is still dogged by complaints that he comes across as a phony preacher, an image fueled by his recent purchase of a $5.4 million North Carolina estate while on the road railing against “two Americas” — one for the wealthy and one for the poor. “Well, I think we know which America he’s living in,” Jay Leno quipped on NBC’s “Tonight Show.” Aides to the former senator insist that the home is not just for his family’s personal pleasure, but will be used as a conference center on poverty issues.

At the beginning of every presidential campaign, it’s easy to dismiss the field as lightweights until they mature in the public eye. But the front-runners in this race are burdened with enough obstacles that the lesser-knowns and unknowns can take heart. Wild fluctuations in the polling being done in the early primary and caucus states suggest that none of the majors are truly catching fire.

Voters might well take a look at the top contenders and decide to keep looking.

Contributing Editor Craig Crawford is a news analyst for NBC, MSNBC and CNBC. He can be reached at ccrawford@cq.com.