It is a good thing the American Constitution calls for instant run-off presidential elections the minute the "winner" dips below 50% approval, or else we would be stuck with the most unpopular president in American history for years to come.
Thankfully Sarah Palin may now take her rightful place as America's "Commanderin' Chief" because Barack Obama is polling at 49% in the daily Gallup tracking poll. As Gallup notes, this is an unprecedentedly fast drop in presidential approval, not counting the three post-war presidents who dropped below 50% faster than him (Reagan, Ford, Clinton) and the one who also dropped below 50% after 11 months in office (Truman).
One of the fastest and most effective ways for any president to boost his approval rating (besides starting a war or botching his response to a terror attack) is to score a major legislative victory. So, with "centrist" and Blue Dog democrats spooked that Obama's low numbers will negatively affect their own reelection bids, look for them to attempt to obstruct and delay any legislation the president champions, in order to drag down everyone's numbers even further in the hopes that eventually elections will just be "called off."
The primary reason for the president's declining approval is, of course, the economy. Specifically jobs. Americans know an economic "recovery" without jobs is no recovery at all, so, once again, look for Blue Dogs and centrists to become incredibly terrified of the idea of spending any further government money on jobs programs of any kind, as we all know that's why FDR was a one-term president, because of his socialism, and how much it made Americans hate him. Jobless Americans are much, much more concerned with the abstract concept of "fiscal responsibility" than they are with material concerns like health care, regular employment, and "having enough money to feed themselves."