Josh Marshall makes explicit something that most observers fail to realize — their judgments of candidates are shaped by political circumstance:

The third is just how weak this field really is — something I knew but hadn’t seen yet quite so up close. I can’t imagine that a sentient Republican could have watched that 90 minutes and not been at least quietly aghast… But the issue isn’t so much that most of them don’t seem up to the challenge of being president. It is more that the political climate and the state of the Republican party in general makes their answers to most questions either off-balance, awkward or completely incoherent.

Conversely, the favorable environment lends the Democrats (perceived) stature, momentum, etc. Maybe it’s an obvious point, but you’ll be hard-pressed to find anyone in the press making it when a campaign gets rolling. Instead, the press tends to manufacture biography-style narratives in which candidates won and lost due to their actions they took and the supposed qualities they possess. Don’t believe it.