Let me wade
into this McCain as a Democrat talk. First, this MSNBC article
points to various interest group score cards and litmus test votes which should supposedly knock McCain out of contention in a Democratic primary. Look closely though because it's a somewhat misleading list. On its face it's damning but most of the votes and scores are lifetime scores or ones that are several years out of date. The AFL-CIO, for instance, apparently gave him a zero rating in 1999.
The problem with all these numbers is that McCain's serious leftward tilt pretty much all comes from during and after his 2000 presidential run. Before that, campaign finance reform was still pretty much his only break from mainstream conservative Republican orthodoxy.
Thus, statements or votes prior to that date simply beg the question of where McCain is politically today. Plus, candidates who've had political conversion experiences need be far less troubled with past votes and actions than conventional politicians.
Mickey Kaus notes talk that McCain is still most likely to run as an Independent.
But if there are good reasons McCain would not or could not run as a Democrat there are even better reasons why he wouldn't or couldn't as an Independent.
Here's the best one: as I told Green while he was writing the article, if you actually want to become president as opposed to run for president, you'd never do it as an Independent because they never win.
Look at the data.
Last time an Independent was elected president since the creation of the two-party system in the 1820s?
It's never happened. Some interesting test cases include the lead-ups to the election of 1844 and perhaps also the election of 1868, when vice-presidents who had become president considered it.
The last time a third-party candidate became president?
Abraham Lincoln in 1860. And the proximate cause was sectional, not ideological, disintegration of the party system.
So, does the McCain as a Dem idea face a lot of hurdles? Absolutely. But the reason McCain would choose to run as a Democrat rather than an Independent would simply be that he is interested in winning.