How it will come down to South Carolina

Barack Obama has 31 field offices open in Iowa, far more than any other Democratic candidate.  As Chris Cillizza of the Fix notes, if (and only if) Obama wins Iowa, independent voters in New Hampshire, who make up 45% of the state's electorate, would be more likely to vote in the Democratic primary.  However, Clinton far ahead in New Hampshire.  So if Obama can win Iowa, finish in a close second in New Hampshire, that would set up a make or break match in South Carolina:

If Obama manages to win Iowa, it's possible a slew of indepedents wouldflock to his cause in New Hampshire where they are allowed to vote ineither primary. That sort of surge among independent voter could erasethe Clinton edge in the state or at least drastically narrow it.Imagine a scenario in which Obama wins Iowa, Clinton narrowly wins NewHampshire and the two square off in South Carolina -- a state whereblack voters made up nearly 50 percent of voters in the 2004 Democraticpresidential primary.

Back to New Hampshire for a bit.  John McCain is hoping that Clinton wins Iowa because more New Hampshire independents would be likely to vote in the Republican primary.  John McCain needs independents in order to stay alive.  However, if Obama wins Iowa, that extra media buzz about the Clinton family getting knocked off would motivate New Hampshire independents to vote in the Democratic primary instead.