I think this interpretation relies on an overwhelming major-party bias in most of our political thinking.
In the two months since Gov. Charlie Crist began building up to and ultimately did switch from Republican to independent, he appears to have overtaken Rep. Kendrick Meek as the de-facto Democratic candidate in the race against Republican Marco Rubio, according to the polls.
Crist is not becoming the “Democrat.” More Florida voters now view the Democratic Party in the same way they do third-party potential spoiler candidates. Often in American politics, a third-party or independent candidate will poll well, only to see support drop off heading into the actual election.
That is because the design of our first-past-the-post election system encourages tactical voting and creates the issue of the spoiler effect. People skip voting for their favorite candidate because they think voting for the second choice is the only way to prevent the candidate they really hate from getting elected. If Meek is seen as unelectable, it is not surprising that anti-Rubio and/or anti-Republican voters would gravitate to Crist as the lesser evil.
We have become so entrenched in our Democratic/Republican two-party political mindset that even when it breaks down, as it has in this race, we try to warp reality to fit the classic narrative. Most Americans view the major parties very negatively. People are unhappy about the last two years under Democratic control but still blame Republicans for the huge problems they caused when they had power. It shouldn't be surprising that a significant group of people wants to vote for non-Democratic, non-Republican alternatives as long as they are viable. We don't see this often because our election laws inherently favor having only two parties.
The success of the major parties has less to do with people having positive feelings about them and more to do with the fact that they have huge institutional advantages. We are not a country of Democrats and Republicans, just a country forced to choose between only those two. That’s when zero-sum politics comes into play. Some people will vote for Democrats simply because it is the only way to vote against Republicans and vice versa. When candidates are able to tear down each other’s favorable numbers dramatically, there is an opening for an alternative--only if voters think the alternative has a chance of winning.
Crist’s success will demand heavily on remaining a viable choice in voters’ minds. If he starts polling in third place, his support will drop rapidly. He needs Meek to be perceived as a spoiler, leaving himself as the only candidate someone could vote for to prevent Rubio from taking office. This is not about Crist becoming the de facto Democrat. It is about him becoming the de facto anti-Republican. The actual Democrat is now reduced to third-party spoiler status in most voters' minds.