FIRST VOTES CAST: Sanders pins Southern hopes on Iowa win

U.S. Sen. Bernie Sanders of Vermont hopes to pull off an upset against front-runner Hillary Clinton in Iowa. The state is a must-win for Sanders, looking to prove his electability to voters in the South, where Clinton dominates in polls.

What the polls say: Hillary Clinton and Bernie Sanders are in a tight race in Iowa, having swapped the lead in polls, sometimes within the margin of error. In an average of polls, Clinton still has a slight edge – 47 percent to 44 percent. That’s a big change from December, when some polls had Sanders trailing by double digits. The shift could mean that Sanders’ efforts to tie Clinton to Wall Street and the establishment are paying off.

Who must win? Sanders. The self-described democratic socialist faces almost certain losses in the South. A win in Iowa followed by New Hampshire, where Sanders is the favorite, could sway voters in South Carolina, where Clinton has a commanding lead. A loss in Iowa will spell trouble for Sanders by undercutting his claim that he is electable.

What’s key: Turnout and Martin O’Malley’s supporters. Sanders is counting on first-time caucus goers, who favored the Vermont independent over Clinton by 51-41 in a recent poll, to turn out in his favor. Democratic caucus-goers loyal to O’Malley, the former Maryland governor who has not escaped single digits in the polls, also could help Clinton or Sanders to victory. Candidates must have enough support from Democratic caucus-goers to be “viable” to qualify for delegates. If O’Malley does not reach that threshold, his supporters will have a chance to back another candidate.

What does it mean for S.C.? Who Iowa’s majority-white Democratic caucus-goers choose on Monday will not be a good measure of how South Carolina’s majority-black Democratic primary voters will vote. But Sanders and some S.C. Democrats say an Iowa win could help prove to Southern Democrats that the senator is electable, shifting some S.C. support away from Clinton.