A big TV buy could help Mr. Sanders with the demographics among which he is weakest: older voters who are the likeliest to be watching television without skipping the commercials.

“They’re doing very well with younger voters,” said Joe Trippi , a Democratic consultant who managed the Vermont governor Howard Dean’s insurgent 2004 presidential campaign. “He’s going to break out among older voters who are not with him, and that’s a TV audience.”

In recent weeks, Mr. Sanders’s campaign has been making it increasingly clear that it views Iowa as critical to his chances of winning the Democratic nomination. It has built up its state team and expanded its ground operation.

It has similarly tried to display strength in New Hampshire: Earlier this week, Shannon Jackson, the state director there, sent a memo to supporters saying the Sanders campaign was “in a strong position to win.” The campaign, according to the memo, now has 14 field offices and 90 staff members on the ground in the state.

Yet while much of Mr. Sanders’s 2016 appeal was predicated on his being the lone alternative to the establishment choice, Mrs. Clinton, in 2020 he is competing with a field of 16 remaining rivals. Only one, former Vice President Joseph R. Biden Jr., has a plausible claim on widespread establishment support.

In Iowa, Mr. Sanders has largely opted out of the cattle-call events that have drawn most of the presidential field, forgoing opportunities to make his case to both undecided attendees and supporters of other candidates who might be persuaded to pick Mr. Sanders as a second choice. In Iowa’s caucuses, supporters of a candidate who fails to attain the 15 percent threshold for earning delegates may reassign themselves to another candidate.

Mr. Sanders bought no tickets last weekend to the Iowa Democratic Party’s Liberty and Justice Celebration, a huge annual political event in the state, nor did he take supporters to the Polk County Steak Fry in September. Both events drew more than 12,000 Democrats, and other campaigns used them as opportunities to demonstrate their organizational might and collect data on caucusgoers who could be persuaded to support their candidate.