Rupert Cornwell: Romney's fate may rest on Illinois result

In this gruelling Republican primary season, many a battle has been termed make or break, only for hostilities to resume the day after with the balance of forces virtually unchanged. But next Tuesday's contest in Illinois could be the exception.

It comes in mid-campaign, when Mitt Romney's frontrunner status looks shakier than ever after third-place finishes in Alabama and Mississippi.

Victory for Rick Santorum in Illinois could be the turning point: the moment when Mr Romney's aura of inevitability is finally dispelled, and when the former Pennsylvania senator emerges decisively as the conservative standard-bearer.

Having narrowly failed in Michigan and Ohio, Santorum must show he can win a rust belt state. Right now, Romney appears just ahead in Illinois, but that advantage may vanish after Tuesday's losses.

He is blitzing the state with TV ads, much as he did in Ohio and Michigan when defeat appeared to loom. But Santorum has momentum and success in a big midwestern state would have a huge impact. It would again set phone lines burning, as jittery party barons – convinced a Santorum nomination would be a disaster – again sound out possible white knights such as ex-Florida governor Jeb Bush. Romney has long been seen as the best bet to beat Barack Obama. Defeat in Illinois may end that.