Mitt Romney's support is being challenged on two separate fronts.
Politically, he is now tied with Michele Bachmann in Iowa. While Romney was always likely to lose a portion of the conservative vote to one of the many candidates to his right, the Minnesota congresswoman is also stealing his spotlight and leaching away his star-powered popularity. Romney is a household name - an advantage he holds over most of his intra-party rivals (now that Trump is out of the race and Gingrich seems to have stalled). But Bachmann is fresh and attractive (politically, I mean) - she has the power to siphon votes founded upon Romney's charm and charisma. She's the only candidate who can compete cosmetically with Romney's "hair factor."
Economically, Romney's critical base of donors among Utah's Mormons is being courted by Jon Huntsman. Romney must be reeling from the statistically improbable appearance of another Mormon in the presidential race. The dueling Mormons have now created a fissure in the Mormon constituency - which is conservative on most issues, but very liberal in campaign donations.
Romney is still a well-funded frontrunner - but that makes him a legitimate target for other Republicans and threatens that his political star may have risen too quickly in electoral time. He's the king of the hill, but Queen Bachmann and the rest of the GOP brood are eager to knock him from his perch. Romney will need to display true political skill if he is to stave off contenders and preserve his elevated stature.