There’s good reasoning in what you say. But the political calculus is like chess; you have to try to perceive things as many moves ahead as possible. Obama’s people may have figured that, among the Republican field, they’d like to go up against anyone but Cain. As many have noted, Romney is perfect because he is despised by the conservative base, cancelling out Obama’s weak standing with the leftist base, and he is neutered on the Obamacare issue by RomneyCare.
On the other hand, Cain may be the guy they fear the most. He’s black, so they have a hard time with the race card. He’s a businessman, so he can talk circles around Obama in terms of real world economics and job creation. He’s tough as nails, so he won’t go mushy under attack by the Chicago machine, he’ll fight back and the public will see David standing courageously up against this corrupt Goliath.
In these terms, one might see that Obama’s best play was to eliminate Cain before he could grab the nomination, because once he’s nominated, Obama is in a helluva fight that not even an October sexual harassment surprise could win. If I’m right, their internal polling and focus group data must be showing them that Cain is very dangerous indeed, which is good news for fans of the Hermanator.
Another reason the Demonrats would want to attack Cain now: Once Cain is nominated, his rivals in the GOP will no longer have much motive for attacking him themselves. By attacking him now, they get at least veiled help from the rivals (notice how Mitt, Gingrich, et al. were lecturing Cain on how “these were serious allegations and he needed to get everything in the open ASAP?). Also, once he is the nominee, he will have access to more resources to fight the smears. Right now, he’s using his own funds.
As I’m sitting here reading through all of this, I am picturing Obommie the Commie and Cain up on stage debating. I can see no scenario where Obama comes out looking like anything but the phony usurper that he is.