If you look back at the last century's worth of Presidents, there seem to be two resume items voters look for: executive experience (state governor or vice president) or significant war service. Even the ones who don't have that have some equivalent executive experience (Hoover fed Europe after WWI, Taft was governor of the occupied Philippines). Having that isn't a guarantee of winning, since the other party usually also nominates someone with a good resume, but the winners have one or both of those items.
The Dems had someone who met that requirement--Bill Richardson--and could've talked some war vets into running (Webb, Kerrey, Clark). Instead they've settled on two senators who've never been in uniform. I suspect either one will be rejected by the middle section of the electorate for not meeting the prerequisites.
Which is a shame. Elections are how we settle things in a democracy. Each one is a chance to take an issue and get the opinion of the American people as a whole, looking at pros and cons. If the vote is on "Is X qualified to handle the presidency?" then there's another four years until we can try to settle an issue. One side of the issue will have lost the election but claim that "The American people still favor our position on ABC. The election doesn't affect that--we only lost because Dukakis/Dole/Kerry/TBD was a lousy candidate." We've got plenty of that already.