Having watched Santorum's speech, I think he is likely the best candidate for the Tea Party. I thought it was excellent and surely will feel coherent for those who think, for instance: that a society is built from the bottom up and not the top down - as opposed to both, which to me seems the sensible view; that Iran is yet another country for the US to have a war with - the US must pay for its cheap oil in blood rather than investing in its chip technology to harness the sun.
If Santorum is knocked off his perch by Romney/Rove attack ads, then Bachmann will be waiting. The right wing Christian vote isn't going to Romney. It won't go to Paul because he won't mortgage America to Israel's interest ahead of Armageddon. It has to go somewhere and it has around 40% of the Republican vote. Where Bachmann stands in the polls is irrelevant. The question is, who owns the bubble.
If she quits she loses her back-up position. But at this point, Santorum has the bubble in his hands. Let's see if he can keep it.