I actually think both do harm and good for the American way. Conservatives (who tend to be Republicans) have a more traditional stance on social issues that go back to the great moments of American History all the way back to the Founding Fathers. Every major event in American History is cherished by the Republicans, and they feel that these are things we shouldn't change (such as "In God We Trust" and God in the Pledge of Allegiance). Plus, they used the idea of "All men are created Equal" to free the Slaves. But, Democrats later on did help with the Civil Rights movement. Conservatives also believe in a more Libertarian Capitalist economy. This is based on the rights to private property and for the government to not interfere with our business.
Liberals (who usually vote Democrat) are a more progressive group, which is really what America is about. They believe that change in Social status is good. Now, I don't agree with most of the things they want to change because it interferes with my civil liberties, i do have to say that the want of progression for our country because they feel it is what's right for us is a good thing. They want to legalize gay marriage which is bringing about more equality to the American people. They want to educate the American people with Science and they want more Separation of church and State which is also what this country is about.
Now, I do feel that in this current state, the Republicans have been better representatives of America (despite some of their Christian enforcement) mainly because what they've been doing so far has been based more on American Tradition and the Constitution.
Although, the party I think represents America the best (despite many of the idiotic idealists within it today) is the Libertarian party. Taking a social liberal perspective and a conservative economic perspective, and, believing in every right given by the constitution, that the other parties sometimes ignore (free speech, Gun rights, privacy, and private property).
me, i just pull out of all the parties and take the ideas i do like from them, Gun rights, private property, Traditionalism from the Republicans, and Progressive thought and right to privacy from the Democrats.