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After months of seeing her polling advantage narrow, Hillary Rodham Clinton has widened her lead against Senator Bernie Sanders, according to a CNN/ORC International poll released on Monday.

Mrs. Clinton drew the support of 42 percent of registered Democrats and independents who lean Democratic, giving her a commanding lead over Mr. Sanders, who had 24 percent. The poll has a margin of error of 5 percentage points.

The numbers should be comforting to Mrs. Clinton’s campaign, which has seen Mr. Sanders overtake her in polls in Iowa and New Hampshire and gain ground on the former secretary of state nationally. A September poll from CNN showed Mrs. Clinton leading the Vermont senator by 10 percentage points.

However, the survey also shows Vice President Joseph R. Biden Jr., who is considering a run for president, solidifying his appeal. Mr. Biden trailed Mr. Sanders narrowly in Monday’s poll with 22 percent support. He is the top second choice for likely voters, although if he chooses not to run the poll shows that the bulk of his backers will shift to Mrs. Clinton.

For his part, Mr. Biden maintains that his personal deliberations for a run are continuing.

“I just have to be comfortable that this would be good for the family,” Mr. Biden said in an interview with America Magazine.