I read a good article about the US Republican party talking primarily about how the party has mired itself in ignorance and anachronism. The article makes the very good point that since historically the Republicans have been the party of the religious right and have been able to count on the vote of fundamentalists they have managed to work themselves into a hole that it is very difficult to get out of because of the two party system. Each candidate has to prove to the party's voters that they are the best and the easiest way to guarantee any given person's vote is to find a person who cares about a very small number of issues extremely passionately and then convince them that you are the best person for those issues. There are plenty of Republican voters who base their whole voting decision on religious fundamentalism and since the other voters have a wide range of ideas the best way to become the leader is to appeal to the one issue voters and be a more extreme fundamentalist than everybody else. You can try to appeal to people on all the other topics but it is extremely difficult to appear to be good on defense, social issues, economics and so on so the most successful candidates are going to try to outdo each other impressing the fundies.
This wouldn't be nearly as much of an issue in a multi party system. If there were a decent number of parties then fielding a wild eyed frothing lunatic is sure to alienate everyone who isn't part of your religion but since you have a simple choice of A or B then even if A is insane a lot of people will vote that way anyhow because they don't vote B. Canada has managed to elect a Conservative party that uses the same sorts of tactics as the Republicans - denial of science, refusal to pay attention to facts, insertion of religious dogma into politics and policy and other atrocities. The advantage we have up north here is that there are three real parties right now and there have been several others in the recent past so voters have more of a choice. If the leader of a particular party is clearly nuts Canadians can and will vote for somebody else to a much greater extent than the Yanks. Because of this Stephen Harper has to downplay the religion and the fundamentalism to keep his support base pacified - there are plenty of people who vote Conservative who would find somebody else to vote for if Harper went too nutty on them. Somehow this didn't stop us electing a Conservative majority who will proceed to do all kinds of terrible things but I suspect this is largely based on the left wing vote being much more divided between the centrist Liberals, the left leaning NDP and the marginal Greens.
Here in Canada the Conservatives could actually field a progressive right wing candidate and do well. Right now they are essentially a despotism unfortunately so we seem stuck with Stephen Harper's egomaniacal behaviour and piss poor policies. It would be completely feasible to have a Conservative leader who was really pro science and who made good, informed decisions and the party could do very well in such a situation. The fundies might vote for them less but the majority would vote for them more in such a situation and they could easily do much better than they have and might well make a very good government. Canada's system of government isn't ideal by any stretch (we need the random vote!) but it is a damn sight better than a historically entrenched two party system which ends up being run by the most extreme people.