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.

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.

Only two, Ronald Reagan and Bill Clinton, turned things around. Both saw their approval ratings drop below 40% early in their presidencies, but each had time to recover and got elected to a second term.

The current President Bush's trouble arrived in his second term. Battered by high gasoline prices, declining public support for the Iraq war and lingering anger over the federal government's slow response to Hurricane Katrina, Bush saw his approval ratings slide to 39% in an Oct. 13-16 USA TODAY/CNN/Gallup Poll, before climbing back to 40%-42% in four ensuing polls. In the most recent poll, taken Nov. 11-13, Bush's approval rating fell to 37%, the lowest the poll has recorded in his presidency.

Though he no longer has to think about re-election, Bush still has three years left in his presidency — time in which he'll work to improve the U.S. position in Iraq, hold the Republican majority in Congress in 2006, restart a stalled legislative agenda and try to build a legacy. Slumping approval ratings will make all of those tougher. Even as he sought the comfort of his Texas ranch for Thanksgiving this week, Iraq war protesters on nearby roads were a reminder of his troubles.