Perhaps the most interesting aspect of a new American Research Group poll of Michigan, which gives Rick Santorum a six-point lead over Mitt Romney, a native son, is the stark divide it shows in the level of voter commitment to the candidates.

Among the Republicans that the polling firm classified as definite voters, Mr. Santorum’s lead was larger, 11 points over Mr. Romney. However, Mr. Romney led Mr. Santorum 33 to 22 among voters the pollsters classified as more marginal.

Ordinarily, a candidate should benefit from having the support of more definite voters — and most polling firms give them the bulk of the weight in their turnout models, which is why Mr. Santorum leads the poll over all.

The universe of indefinite voters is broader. But those votes don’t count for anything unless the candidate can get the voters to the polls.