DES MOINES — Martin O’Malley has rarely broken above 5 percent in Iowa polls, but on caucus night he could be the most popular person in the room — or, rather, his supporters will be, as activists for Hillary Clinton and Bernie Sanders desperately try to scoop them up.

The arcane rules of Iowa’s Democratic caucuses mean that most O’Malley supporters will be ruled “nonviable” if he does not get 15 percent support at a caucus; his supporters will then be up for grabs by another candidate. With polls showing the race between Mrs. Clinton and Mr. Sanders narrowing to a near tie, O’Malley supporters, along with attendees who enter their neighborhood caucuses undecided, could swing the results.

“The most coveted person in Iowa politics is an undecided person or the O’Malley nonviable folks,” said Kevin Geiken, a Democratic strategist in the state.

Both the Clinton and Sanders campaigns, which have spoken to nearly every potential caucusgoer in the state by now, know who is supporting Mr. O’Malley and, more important, who their second choice is. After an initial count at each of the 1,681 caucus sites, supporters of Mrs. Clinton and Mr. Sanders will get a chance, in an often emotional and chaotic scene, to woo O’Malley supporters if they make up less than 15 percent of the people in the room.