For the first time, Barack Obama fell below 50 percent in the Gallup tracking poll.

Obama's decline comes at an historic pace. It ranks in the lower third of modern American presidents.

The fall below 50 is a fraught milestone for any president. In legislative terms, a president is only as powerful as he is popular. Public approval rating is the metric of that popularity.

Below 50, a president can no longer claim the majority's support. His political arsenal depletes. A president's political opposition has powerful, though nebulous, new ammunition.

Of the twelve presidents since World War II, Obama lost the majority at the fourth fastest rate. It nearly was worse. Ronald Reagan's fall-point came about a week sooner. That Reagan also fell below 50 during his first November, and would go on to languish below 50 for two years, illustrates that the milestone is not determinative.

But Friday's news—Obama at 49 percent—is indeed symbolically significant. It will immediately impact the health care debate, albeit intangibly and perhaps only slightly. It will be more difficult for the White House to pressure moderate Democrats.

Obama owes most of this year's decline to independents. But Obama's newly weak standing is also due to the loss of still-more Republicans and to a lesser extent, some Democrats.

The physics of Obama's approval rating generally reflect George W. Bush's in 2004, when he first dipped below 50 (mid 80s with his party, low to mid 40s with independents and high teens with the opposition). Bush below 50, however, maintained slightly stronger support within his base than Obama—possibly because 2004 was an election year.

Clearly, Obama's greatest issue is with the fickle middle. Obama's fragile bond with independents has been visible throughout his young presidency. As early as mid March, this writer was wondering: Can Obama Hold the Center?

Other recent polls, like Quinnipiac, have also for the first time tracked Obama below 50. But Gallup has unrivaled historical reach. Its findings carry unique symbolism. Obama's RealClearPolitics average, the mean of major public polls, is at 50.6 percent.

The bulk of Obama's public opinion decline occurred over the summer. Democrats can largely blame the crawling health care legislation. But more recent issues have not helped, including double-digit unemployment and his unpopular decision to hold the trial of Khalid Sheikh Mohammed in civilian criminal courts.

All presidents, save John Kennedy, eventually fall below 50. Gerald Ford fell below 50 during his third month (pardon of Richard Nixon) and Bill Clinton in his fourth month (a confused and unpopular early domestic agenda). Reagan fell in November 1981 (the U.S. was entering, what was then, the worst recession since the Great Depression).

The remaining post-war presidents held the majority's support longer than Obama. Excluding Kennedy, Obama's predecessors lost the majority on average after 23 months in office; the median was 13 months.

For some presidents, the fall below 50 fluctuates but generally marks the start of an unrelenting decline (Gerald Ford and Jimmy Carter). Other presidents languish in the minority but eventually recover (Reagan and Clinton). Some presidents fall below the threshold and quickly recover (Dwight Eisenhower).

Democrats need not panic. Most modern re-elected presidents fall below 50 at some point in their first term. But his early popularity (nearly at 70) means that few have, so quickly, fallen as much as Obama. Obama's public approval suffered the worst third quarter (July 20 to October 19) decline of any elected president in the post-war era, according to Gallup.

In a time of tracking polls, Obama can recover and fall in a matter of days. After W. Bush fell below 50 in January and February 2004, he fluctuated in the 40s and low 50s throughout the year. Bush's long dive began in his second term.

Obama first saw 50 in late August, according to the Gallup tracking poll. We've known since then, with an eye on how other presidents dealt with that milestone, that: For Obama, The Fall Below 50% Looms.

Now Obama is here, a president without the majority. I've written before that public approval is the currency of political capital. Now we'll witness—as Democrats struggle to extricate health care from its legislative quagmire—how Obama governs without the riches of what was, last January, soaring public support. These are the periods that test presidents.