Just over three months before Iowans caucus for Democratic presidential candidates, four polls out this week present a muddled picture of a primary race that is unfolding in varying, and sometimes conflicting, ways nationally and in early-voting states.

A pair of new national polls present starkly different results. A CNN survey released Wednesday had former Vice President Joseph R. Biden Jr. with a commanding lead of 15 percentage points ahead of Senator Elizabeth Warren of Massachusetts. But a Quinnipiac University poll released Thursday had Ms. Warren as the front-runner, seven points ahead of Mr. Biden.

Senator Bernie Sanders of Vermont finished a strong third in both national polls, suggesting that his standing in the race has not been diminished by his heart attack earlier this month — and that he may have been buoyed by endorsements from Representatives Alexandria Ocasio-Cortez of New York and Ilhan Omar of Minnesota.

Two other polls from early-voting states show a tight race in Iowa and Mr. Biden maintaining a comfortable lead in South Carolina.