Democratic strategists and supporters of President Obama are concerned that Howard Dean will become a relentless and hostile liberal advocate who could rally the Democratic left against the White House over the next three years. Some are even concerned that Dean, former chairman of the Democratic National Committee, ex-governor of Vermont, and past Democratic presidential candidate, might challenge Obama for the Democratic nomination or run as an independent candidate in 2012. Sources close to Dean say he feels badly treated by the Obama administration, partly because he was not named secretary of health and human services even though he is a physician who has a lengthy history of working for healthcare reform and was a respected party chairman. But Democratic sources say Dean is considered too much of a maverick and a loose cannon by Obama insiders. And as DNC chairman, Dean tangled over funding and strategy with Rahm Emanuel when Emanuel was a U.S. representative from Chicago and headed the Democratic congressional campaign team. Now Emanuel is White House chief of staff, and Dean allies believe it was Emanuel who blocked his appointment as HHS secretary. In recent weeks, Dean has strongly criticized the healthcare legislation passed by the Senate and advised Congress to scrap that measure and start over. Dean says the Senate bill doesn't go far enough in reforming the system. He is also expected to become more active in leading a liberal political action committee on a variety of issues. A Democratic strategist who is close to the White House says that President Obama believes there will be plenty of opportunities to work with Dean in the future. But the strategist concedes that there are important issues on which Obama and Dean will disagree.