Mitt Romney is in a prime position heading into the Iowa caucuses tonight. But even though he’ll almost certainly finish in the top three, that doesn’t mean he can’t “lose.” Obviously, the best case scenario is Romney takes the top slot, and the second best is he finishes second to the untenable Ron Paul. A slightly worse outcome is if Romney comes in second to Rick Santorum, and the losing scenario is if he finishes in third, behind both of them.

The Washington Post sums up the impact a third-place showing would have on the Romney campaign:

If he places second to Paul or Santorum, Romney would still have outperformed expectations, and the narrative out of Iowa would be that he is poised to make a strong showing in the upcoming nominating contests.

But a third-place finish in Iowa – particularly since Romney has gained so much in the polls — could prove problematic.

It would fuel the argument that Romney is not the top choice for many conservatives, who still distrust Romney’s record as governor of Massachusetts. It would shatter Romney’s air of inevitability. And it would likely give his rivals a foothold in fundraising, which could mean a lengthier primary fight than originally expected.

Romney’s recent rise in the Iowa polls was unanticipated, but now that his campaign has set the expectation he’ll finish in the top two, anything short of that will be seen as somewhat of a failure. It wouldn’t be a crisis for his campaign by any means, but it would portend a longer, drawn-out primary and undermine the perception of inevitability that Romney’s been building back up.

The less problematic, but still not ideal, scenario for Romney would be if he came in second behind Santorum. While a Paul victory in Iowa would likely discredit the caucuses altogether, Romney would have to take Santorum more seriously as a challenger. It could mean sparring with Santorum at the upcoming debate, potentially sinking more money into attack ads, competing seriously in states Romney otherwise might not have to worry about, and, above all, drag out the primary battle.

That’s not to say the race is over if Romney wins the top spot tonight. But it would create a snowball effect of inevitability, especially heading into New Hampshire, which Romney already has locked up.