New polls published on Wednesday show Joseph R. Biden Jr. with a significant national lead in the Democratic presidential race as the contest heads into the intense fall campaign season, with Senators Elizabeth Warren and Bernie Sanders rounding out a threesome that seems to have separated from the rest of the primary field.

The new data, which shows Mr. Biden earning the support of about one in three Democratic voters, stands in contrast to a poll published Monday that showed a virtual three-way tie between Mr. Biden, Ms. Warren and Mr. Sanders at about 20 percent support each. On Wednesday, the director of that poll called those results an outlier.

The latest round of polling suggests that Mr. Biden’s national lead — at this early stage in the campaign — has been relatively durable throughout the summer. It has been fueled in part by the former vice president’s strong name recognition, calculations about general election viability and good will from rank-and-file Democratic voters, even as there are signs of greater volatility on the ground for Mr. Biden in the early-voting presidential primary states.

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Still, national polls can offer a snapshot of the Democratic electorate’s mood, and of the extent to which various controversies have caught the attention of voters. One of the polls published Wednesday, conducted by Quinnipiac University, showed Mr. Biden as the top choice of 32 percent of Democratic voters and independent voters who lean Democratic. Ms. Warren came in at 19 percent and Mr. Sanders was at 15 percent.