<p>When a president falls below 40% approval in public opinion polls - as President Bush has done twice in the past two months - it's usually a sign of serious political danger.</p>
<p>Since 1950, five of the eight other presidents who fell below 40% - Harry Truman, Lyndon Johnson, Gerald Ford, Jimmy Carter and George H.W. Bush - lost their bids for re-election or opted not to run again. A sixth, Richard Nixon, was overwhelmed by the Watergate scandal and resigned.</p>