Vice President Joe Biden has less support in the polls than Bernie Sanders and hasn’t raised a single dollar for a presidential campaign. Yet if Mr. Biden does decide to seek the presidency, he will pose a greater challenge to Hillary Clinton for the Democratic nomination.

In Mr. Biden, Mrs. Clinton would have an opponent who could threaten her hold on the coalition of moderate voters and party elites that seems to have the advantage in this race over the party’s white, liberal activist wing, which now supports Mr. Sanders.

Mrs. Clinton would remain a clear favorite even if Mr. Biden ran. But unlike Mr. Sanders, the Vermont senator who has made gains largely by winning over those already inclined to be unhappy with Mrs. Clinton, Mr. Biden would have strength that would come almost exclusively at her expense.

According to polls from the last month, Mrs. Clinton is the second choice of nearly all of Mr. Biden’s supporters. His growing strength since July, when he began to flirt with another presidential run and when scrutiny intensified over the private email account Mrs. Clinton used while secretary of state, is largely responsible for her decline in the polls.