Ever wonder if someone is a liberal? There's a fool proof test to know for sure. A person's reaction to simply the word liberal is all you need to know. If the person wishes to distance himself from the word liberal then he's not a liberal (well one exception, when the person is running for office). If the person says something to the affect of, "I'm not, but what's wrong with being a liberal?" or openly embraces the label, he is a liberal.
So now that you know who's a liberal, now you need to know why liberals are dangerous. They are dangerous for the same reason "right wingers" are dangerous. Being an extremeist is never good. Case and point example religion. Look not farther then bombed out abortion clinics and the big hole in the ground that used to be the world trade center. In America, political extremeists may not kill people (although I know of some people who would claim Bill Clinton has), but they will do whatever else they can to further their positions.
The most dangerous part of being on the political far left and far right in America is the affect it has on this country's unity and reputation. The most common attack on the unity of the country is the president being acted by the opposing party. Bill Clinton, for all his faults, was wrongly accused of using the military action in Kosovo to divert attention from his impeachment. George W. Bush, for all his faults, was wrongly attacked for those words in the State of the Union about Iraq trying to buy yellow cake from Africa. These tactics of attacking the President, whether true or not, polarize the country and a polarized country does not progress.
Another attack on the unity of America is the issue of patriotism. The right wing waves flags to a fault and drop F bombs on people they disagree with on the senate floor (sorry I hate Cheney, couldn't help myself). Anyone who doesn't fully support the war in iraq or say nothing but "Bush is a great leader" are unpatriotic. Liberals on the other hand act more often like they wished they lived in France. A proclaimed liberal who I work with says his favorite rendition of the national anthem was by Roseanne and he would dodge a draft if faced with that situation.
As for the country's reputation, Michael Moore and the rest of Hollywood types continue to bash America in European press. While the right continuely discredits the United Nations and all other countries who have disagreements with American pro-war policies.
Bottomline, be open minded. No one is always right. The choice is clear, be a moderate.