1) Liberals call out and vilify their extremists. The Conservatives ELECT them. This is a major difference. There is no liberal equivalent in national politics of Palin, Cruz, Bachmann, or Christine O'donnell.
So first off you saying Ted Cruz is more extreme then Liz Warren? Why is Ted Cruz extreme? Because he want's to control the size of the federal government using the tools made available by the constitution (congress is not intended to rubber stamp the presidents spending wishes). And comparing Cruz to a Senator who says that a business owner did not build their business, it was government that did it.
Michelle Bachmann - no longer in congress. Why? Because she would have been primaried due to being wacko.
Sarah Palin - Hasn't won an election since 2006; hasn't run for office since 2008.
Christine O'Donnell - (you're stretching by using her). Hasn't been on a ballot since 2010; and the main reason why she won the primary is because Mike Castle was a Liberal Republican.
The thing is, Nancy Pelosi, the Democratic leader in the House is more extreme then any of the Republicans you mentioned.
It's just only conservatives can be extreme, not liberals. Proof: The Tea Partiers vs the Occupiers. The Occupy movement is a lot more extreme then the Tea Party movement, yet is not called out as such.
2) The Democrat party accepts moderates and centrists, while the Republicans kick them out.
Who are these moderates and centrists you speak about and where are they? They are no longer in the House as they were wiped out by the Republicans in 2010 and 2014. Why? Because the party has tilted so far to the left that the populance elected more moderate republicans instead. Also, if you don't accept the Dem party line on abortion and gay marriage, then you are no longer accepted in the party.