Actually they didn't vote against impeachment. The impeachment vote was taken in the House of Representatives. The impeachment trial was conducted in the Senate. At that point, nobody could vote for or against impeachment as the vote had already been taken to impeach clinton. Warner and Thompson both voted against CONVICTION on the first article, and both voted Guilty on the second. In my opinion the perjury charge was the blatant open and shut article that should have warranted a guilty vote, but then again I did not have the opportunity to sit in the Senate and hear the case tried. Thompson has explained why he voted the way he did in the impeachment trial, and his explanation was satisfactory as far as I'm concerned. It showed that he took his duty and oath seriously to evaluate the case and vote based on the merits rather than on partisan politics. Clearly the democrats in the senate did not do so on either article.
Legally, the perjury article of impeachment was far from “open and shut.” In fact, according to Thompson:
“Never has the Senate convicted on an article worded such as this. Several crimes or categories of crimes (the exact number cannot be determined from reading the article) are charged in this one article. The perjurious statements are not described, nor are their dates. In large part, this article charges that the President committed perjury because he denied prior perjury. At the outset, it is clear that a count such as this in an indictment would not survive court challenge...”
Had the House of Representatives drawn up a perjury article that would have withstood the test of law, I believe Fred would have voted for it. He voted for the obstruction of justice article.
http://fredthompsonfaq.com/tiki-view_faq.php?faqId=1#q24