In short, it shouldn’t be this way, but, as of today, the only way Romney becomes president is for him to win a bloody primary war and then have the economy be perceived as being in complete collapse. Even then, the general election would probably still be very close.
A more likely scenario is that Santorum continues to zoom past Romney, Gingrich finally (in the ultimate act of revenge on Romney) drops out, and Romney, stripped of his cloak of inevitability and electability, is toast. As impossible as it may be to believe (and much to the chagrin of Tim Pawlenty, who easily could have been in this position) Rick Santorum, who has spent years being largely shunned by the conservative movement, will likely be the Republican nominee for president.
Santorum, of course barring a catastrophe for Obama, would not win in November. He would be easily portrayed as too green, too extreme on social issues, and too prone to saying “wacky” things, to defeat Obama unless the president was all but a mortally wounded.
There are many people/elements to blame for this tragedy of the chance to limit Obama to one term being allowed to slip through our fingers. Here is a preliminary and partial list.