Where could such a charge of fraud have started?

Scott Brown, a former Massachusetts senator and a Trump supporter, told Boston Herald Radio in November that 100,000 Massachusetts residents had voted in neighboring New Hampshire that month, and that 40,000 had done so when he ran unsuccessfully for the Senate in New Hampshire in 2014. “People from Massachusetts could have gone up and voted that day, and quite frankly some do,” Mr. Brown said, offering no evidence. “It’s well-known, it’s no secret.”

Does New Hampshire have a history of this sort of thing?

Some Republicans have long claimed that New Hampshire’s same-day registration law, which lets people register on the day of the election, has allowed nonresidents, especially college students in neighboring Massachusetts, to vote. The law requires they show valid identification at the polls.

If they do not have such an ID, their picture is taken and they have to sign an oath swearing to their identity and that they are voting legally. After the election, the secretary of state sends out confirmation letters to those voters, who must respond. So far this year, the secretary of state’s office has reported no indications of widespread fraud.

What do other Republicans think?

Lots of them have emphatically rejected the whole notion of voting fraud, even though some have questioned the state’s same-day registration rules. Steve Duprey, a former Republican state chairman and current national Republican committeeman, wrote on Twitter: “Repeating: there is no voter fraud in NH. None. Zip. Nada. Hundreds of lawyers, poll workers, watchers, press -no buses rolled in.” Thomas D. Rath, a former state attorney general, tweeted: “Allegations of voter fraud in NH are baseless, without any merit — it’s shameful to spread these fantasies.” And Fergus Cullen, a former Republican state chairman, wrote: “I will pay $1000 to 1st person proving even 1 outofstate person took bus from MA 2 any NH polling place last ElectionDay.”

What about New Hampshire’s Republican governor?

Before the election, Gov. Chris Sununu alleged voter fraud, saying that out-of-state Democrats were being bused in “all over the place.” Asked for evidence, he modified his claim, saying, “I don’t want to imply I see buses coming over.” PolitiFact New Hampshire said Mr. Sununu’s assertion was “ridiculous” and rated it “Pants on Fire.”

But unlike Mr. Trump, Mr. Sununu won the state and so for obvious reasons wants to be cautious about asserting fraud. “I’m not aware of any widespread voter fraud in New Hampshire; however the administration has made some strong assertions to the contrary,” he told WMUR.com on Monday. “I take their concerns seriously and look forward to working closely with them to learn of any evidence they may have and help address any concerns that may develop from there.”

And Democrats?

Not surprisingly, top New Hampshire Democrats roundly reject the claims. “There is absolutely no evidence whatsoever of major voter fraud in New Hampshire’s elections,” said Senator Jeanne Shaheen. “President Trump continues to spread a dangerous lie, and it’s long past time for Republican leadership in New Hampshire to stand up and defend our state’s electoral system.” And Ms. Hassan’s spokesman, Aaron Jacobs, said the “completely unsubstantiated claims about voter fraud have been debunked by independent fact checkers time and again.”