A big conservative bloc does not automatically translate into a big Republican vote. Republicans managed to lose elections in 1992 and 1996, and lose the popular vote in 2000, even though conservative voter identification in those years reached 43 percent, 40 percent, and 40 percent, respectively.
Obviously, 40 percent of the vote is not a winning margin in a two-party system. Even if we assume that every single one of those self-described conservatives votes Republican, the GOP still needs to pick up another 10 points or so from moderate voters. Ironically, it is the very strength of conservatism that makes it more difficult for Republicans to appeal to moderates. The Democrats know that their core group, liberals, is relatively small. Better still, the liberals know it, too. Sensing their own weakness, liberals allow their party to make compromises: to position itself closer to the center, to say things that appeal to moderate and conservative ears.
The large conservative bloc within the GOP feels no such need for compromise. It feels free to chase moderates away. Who needs them? The result is to divide the American political system between a cohesive and highly ideological Republican Party and a less ideological but larger Democratic coalition extending from the left through the middle.