The Tea Party might be the catalyst that pushes Republican market share among conservatives higher than 80% of the voting electorate. On a national level, that's the single most important metric that strategists are looking at. The relative proportion of conservatives in the electorate is also quite significant, as is the share of self-described moderates. In 1984 and 1994, two wave election years for Republicans -- one a midterm and one a presidential -- 36% of the electorate described themselves as conservatives. In 1984, amid the "great ideological sorting," 70% of conservatives voted for Republicans, up from 65% in 1982, when only 28% of the electorate characterized themselves as conservatives. In the 1990s and 2000s, the closer to 80% Republicans get, the better they do.
The percentage of people considering themselves "moderates" swings a lot, but in the bloodbath elections of 2006 and 2008 respectively, only 78% and 75% of self ID'd conservatives voted Republican. When Republicans were in power in Congress, that number was routinely 80% or above. Democrats benefited from fractional but significant increases in the proportion of the electorate that considered itself liberal.
Gallup notes that in 1994 and 2002, the Democratic share of the electorate dropped below the percentage of adults who identify as Democrats.
There are two ways to assess these correlations. One is that the center holds and the composition of the electorate itself is fixed and not related (or not influencing) these shifts in ideological composition of political parties. Democrats sure hope this is the case, because their midterm strategy is based on it. But another theory, as political scientist Robert Replogle suggests, gives ideological entrepreneurs a more active role. "Ideologically committed political activists," he writes, can "move the whole grid of political opinion in their direction by shaping the alternatives presented to the electorate." That is, the percentage of conservatives who vote Republican depends on the choices available to conservatives, just as the way liberals perceive the electorate depends on how politically energetic liberals behave.
The ideological sorting of the American electorate is basically at an end now, and demographic changes are beginning to exert more influence. The future demographic composition of the electorate -- less white, for one thing -- is an indicator of long-term success for Democrats.
But so long as there are enough white conservatives, and, in off-years, older white conservatives, and so long as Republicans can ways of exciting them, Republicans will remain competitive in congressional elections where less than half of eligible Americans turn out...and less so, relatively, in presidential elections, when the composition of the actual electorate matches the composition of the potential electorate.
Why did Democrats take a beating for passing a health care bill that was very similar in form to what Republican intellectuals had been urging for more than a decade? Because the Tea Party, conservative independents and Republicans have moved the political center to the right--marginally on a 0 to 100 scale, but enough to tip the scale away from Democrats. The electoral environment favors economic libertarians, and the Tea Party movement (or the conservative movement) has organized itself in such a way that really excites conservatives, while liberals, at a disadvantage ideologically (in the sense that conservatism has always been more organized and less diverse) cannot, as they did in 2008, build a tent around a larger coalition.