President Obama won re-election with the rock-solid support of what has become known as the "Obama Coalition" -- young people, minorities, women, and low-income voters. Without a firm foundation -- and high turnout -- among those groups, Obama would not be in the White House today.

Now, little more than a year after the president's re-election, his job approval rating has fallen among all segments of the American electorate. But it has fallen the most among those who did the most to elect him.

For example, according to a new Gallup compilation, Obama's job approval rating among Hispanic Americans has plunged from 75 percent in December 2012 to 52 percent today -- a drop of 23 percentage points, the sharpest decline among any voter group. Among Americans who make less than $24,000 a year, the president's approval rating has fallen from 64 percent last December to 46 percent today. Among Americans 18 to 29 years of age, it has fallen from 61 percent to 46 percent. Among women, it has fallen from 57 percent to 43 percent.

The only key part of the Obama Coalition that did not experience a double-digit drop in support for the president is black Americans, although his support is down there, too -- from 92 percent last December to 83 percent today. Other groups among whom Obama's fall has been relatively minor were those voters -- Republicans, conservatives -- who didn't like him in the first place.

For Democrats, a successful Obama second term, followed by the election of a Democrat to the White House in 2016, depend on keeping the Obama Coalition together. "The primary strategic question for supporters of progressive values and policies is whether this coalition can be sustained going forward and, if so, how it can be harnessed to achieve progressive policy victories," wrote strategists Ruy Teixeira and John Halpin in a post-election analysis for the liberal think tank Center for American Progress. That view is shared by many other Democratic analysts. But the Gallup numbers suggest the president's coalition is already falling apart. That could be a critical development for the next three years, and could have a huge effect when the next presidential election comes around.