WASHINGTON — Hillary Clinton tried. So did 16 rival Republicans. And after hundreds of millions of dollars were spent on ads attacking Donald Trump in 2016, the results were the same: They never did much damage.

Now Michael R. Bloomberg is trying — his way — spending millions each week in an online advertising onslaught that is guided by polling and data that he and his advisers believe provide unique insight into the president’s vulnerabilities.

The effort, which is targeting seven battleground states where polls show Mr. Trump is likely to be competitive in November, is just one piece of an advertising campaign that is unrivaled in scope and scale. On Facebook and Google alone, where Mr. Bloomberg is most focused on attacking the president, he has spent $18 million on ads over the last month, according to Acronym, a digital messaging firm that works with Democrats.

That is on top of the $128 million the Bloomberg campaign has spent on television ads, according to Advertising Analytics, an independent firm, which projects that Mr. Bloomberg is likely to spend a combined $300 million to $400 million on advertising across all media before the Super Tuesday primaries in early March.

Sen. Bernie Sanders frequently attacks Amazon on the campaign trail, vowing to break up the online retail giant if elected, denouncing its treatment of workers and decrying its ability to pay no federal taxes on billions in profits.

That’s why impassioned dissent erupted within the Sanders campaign earlier this year over its purchases of large amounts of supplies through Amazon, according to five people with knowledge of the situation. Carli Stevenson, then a senior Sanders campaign aide in New Hampshire, raised concerns directly with national campaign officials and suggested using a different company, three of the people said.

Another New Hampshire staffer, state political director William Bateson, also voiced opposition to using Amazon, according to two of the people with knowledge of the situation. Stevenson and Bateson declined to comment for this story.

The Sanders campaign continued using Amazon despite the concerns. Through September, it had spent more than $233,000 on Amazon purchases — much of it in office supplies, and often through Amazon’s Marketplace feature — a review of campaign finance records shows.

FAIRFIELD, Iowa — Joseph R. Biden Jr. backtracked on Saturday from his stated position that he would not comply with a subpoena to testify in President Trump’s impeachment trial in the Senate. Instead, he declared that he would abide by “any subpoena that was sent to me” even as he insisted there was no justification for calling him as a witness.

A day after reaffirming that he would not comply with a subpoena, Mr. Biden tried twice on Saturday to clarify his remarks, asserting that there would be no “legal basis” for such a subpoena but left it unclear, for much of the day, if he would ultimately comply with one. Then, questioned by a voter about the issue of compliance with subpoenas, Mr. Biden answered unequivocally.

“I would obey any subpoena that was sent to me,” he said at a town hall-style event in Fairfield.

Mr. Biden’s 180-degree turn on whether he would comply with a subpoena was one of the starkest and swiftest reversals by a candidate in the Democratic primary campaign, and came after he faced questions and criticism about whether his initial stand would run counter to the rule of law.

Mike Bloomberg’s rapidly expanding presidential campaign is moving to Times Square, setting up its headquarters at the old New York Times building on West 43rd Street.

The relocation — from the building that historically housed Bloomberg Philanthropies — comes as the billionaire self-funder and former New York mayor’s campaign pushes past 300 staffers at his headquarters and more than 200 more now active in the states, officials confirmed.

Bloomberg’s new offices, first reported by POLITICO and part of an email he planned to share with staffers Monday, includes an open layout with a desk out in the open for Bloomberg, a move in keeping with his ethos as a businessman and mayor.

In another throwback move, the space spread out across the entire eighth floor (two floors above the first-through-sixth floors owned by presidential son-in-law Jared Kushner) will also feature several countdown clocks showing the days, hours, minutes and seconds to Super Tuesday and the General Election — a nod to his unorthodox strategy to skip the first four states.

Senator Elizabeth Warren’s slip in the 2020 primary polls has been accompanied by a dip in donations, with her campaign setting a rare public goal: aiming to raise $20 million for the fourth quarter of 2019 ending Tuesday, or about 20 percent less than what she raised in the previous three-month period.

Former Vice President Joseph R. Biden Jr., meanwhile, has rebounded from a weak third quarter, in which he raised only $15.7 million and spent $2 million more than he took in. Now his campaign is trying to assert his front-runner status in the Democratic primary, pushing in the final 48 hours of the year to post “our biggest fund-raising quarter yet,” as Mr. Biden wrote in an email on Sunday, by topping the $21.5 million he raised last spring.

The shifting financial fortunes of Ms. Warren and Mr. Biden illustrate the unsettled nature of the Democratic presidential contest heading into 2020, with four candidates — Mr. Biden, Ms. Warren of Massachusetts, Senator Bernie Sanders of Vermont and Mayor Pete Buttigieg of South Bend, Ind. — battling for position in the top tier of polling and seeking to bolster their treasuries ahead of the final sprint to the Iowa caucuses and beyond.

Mr. Sanders is expected to remain a financial pacesetter in the 2020 contest. He is pressing toward a goal of five million contributions by the end of the year, having already passed 4.5 million. If Mr. Sanders hits that goal — and maintains the current average size of his donations, $18 — he will have collected roughly $28.5 million in the fourth quarter, more than any Democratic candidate has raised in any quarter this year.

They entered the race late, but the two billionaires seeking the Democratic nomination are making up for lost time.

Together, Tom Steyer and Mike Bloomberg have poured nearly $200 million into television and digital advertising alone, with the former New York mayor spending an unprecedented $120 million in the roughly three weeks since he joined the presidential race. That’s more than double the combined ad spending of every single nonbillionaire in the Democratic field this year.

“We’ve never seen spending like this in a presidential race,” said Jim McLaughlin, a Republican political strategist who worked as a consultant for Bloomberg’s mayoral bids in New York. “He has a limitless budget.”

The question isn’t whether anyone else will come close to matching Bloomberg's or Steyer’s ad spending. Rather, it’s whether all that spending is making any difference.

At present, the two remain mired in single digits in the polls. Steyer isn’t spending at the same stratospheric levels as Bloomberg, yet with $83 million in ad buys so far, he’s still far outpacing everyone other than his fellow billionaire. The next highest spender on ads is Pete Buttigieg at $19 million.

MANCHESTER, N.H. — A norovirus sickened 18 people after a private function last month at a restaurant co-owned by a Democratic congressman that is popular with presidential candidates campaigning in the New Hampshire primary, state health officials said.

People who became ill attended the private function Nov. 24 at the Puritan Backroom conference center in Manchester, the state Department of Health and Human Services said Friday. One person later died, but it was unclear if the death was related to the norovirus outbreak, the department said.

The restaurant is co-owned by U.S. Rep. Chris Pappas and is famous for its chicken tenders. It has been in his family for more than a century and is a frequent stop for presidential candidates of both parties.

The nature of the event was not released, but it didn't appear that any current presidential candidates attended.