Tancredo's got some very, very, shady backers who have promoted eugenics (for a good takedown of eugenics, read Michael Crichton's site). SJ Reidhead has outlined the very questionable background of some immigration hard-liners at Subway Canaries (most recently, there was a two-part discussion: Part I and Part II).
When we broke away from England, we told the world why. In the document explaining so, Thomas Jefferson wrote the following:
We hold these truths to be self-evident, that all men are created equal, that they are endowed, by their Creator, with certain unalienable Rights, that among these are Life, Liberty, and the pursuit of Happiness.
That's from the Declaration of Independence. If you will, it lays out some of the first principles that this country is founded on. They're very simple ones, if you ask me. Either you believe in them, or you don't. And I am coming to the conclusion that certain people - Pat Buchanan and Tom Tancredo among them - seem to disagree with those principles. Others, like Michelle Malkin, seem to have no problem citing those folks when they make their arguments.
Eugenics is based on the notion that certain races and people are somehow better than others due to their genetics. How that is compatible with the first principles of this country is something I can't figure out. But in addition to its apparent incompatibility with the first principles of this country, eugenics is morally wrong - and its fruits have included some of the worst of human history.
I will have no part of any movement that finds this acceptable. If conservatism chooses to accept Tancredo as a reasonable "mainstream" voice, then I'll have no part of it, then it has left me.