It's true. Of the Dem candidates, only Clark and Kucinich have supported an amendment to the U.S. Constitution allowing Congress to prohibit desecration of the U.S. flag. Kerry, Gephardt, Lieberman, and Edwards have all opposed such an amendment.
http://www.latimes.com/news/nationworld/politics/wire/sns-ap-clark-flag-burning,1,5150155.story?coll=sns-ap-politics-headlines
Why hasn't Howard Dean taken a position on this pressing issue?