Mr. Sanders has few options at this point. Barring a scandal or another extraordinary event that consumes Mrs. Clinton, his only way to beat her starts with a lightning strike: winning the first-in-the-nation Iowa caucuses on Feb. 1 and then the New Hampshire primary on Feb. 9. Such victories would instantly raise questions about Mrs. Clinton’s strength and electability and provide political momentum, fund-raising energy and far greater visibility for Mr. Sanders in the next major contests, in states like South Carolina, Colorado and Texas, where he is not well known.

But Mrs. Clinton has been ahead in Iowa for three months, while Mr. Sanders has a slight lead in New Hampshire polls. Mrs. Clinton held a lead in Iowa in 2007, too, before losing there to Mr. Obama. But he had far more endorsements from state leaders and a sharper line of attack against Mrs. Clinton (over the Iraq war) than Mr. Sanders has. Mrs. Clinton is vulnerable this time around on her ties to Wall Street, and there is also an opening for a candidate to run to the left of her on national security issues.

But Mr. Sanders, who has ruled out negative campaigning, has not done anything memorable on either front. When he did challenge her on Saturday, accusing her twice of being “too into regime change” to topple dictators and enemies, he did it as respectfully as possible — after which Mrs. Clinton hit him hard both times for voting to oust Col. Muammar el-Qaddafi, the former Libyan leader. Mr. Sanders could have brought up the attacks in Benghazi, Libya, that occurred on Mrs. Clinton’s watch as secretary of state, but instead he all but surrendered, saying that Mrs. Clinton was “right” that dealing with dictators was a “complicated issue.”

On Wall Street, meanwhile, he said that corporate chief executives “may like” Mrs. Clinton as president but would not like him — a contrast that could have been made more sharply.