Sen. Bernie Sanders and his boosters are intensifying their courtship of convention delegates who could determine the winner of the Democratic presidential nomination, prompting some party leaders and supporters of front-runner Hillary Clinton to claim harassment.

The Sanders campaign says it has no connection to the efforts of outside supporters to lean on superdelegates, the party leaders and elected officials who can cast nomination votes for any candidate and who are seen as increasingly pivotal in the Democrats’ unexpectedly drawn-out nominating contest.

Among those efforts is a website created last week under the name Superdelegate Hit List, providing phone numbers and addresses for superdelegates and encouraging users to submit further contact information, presumably to help advocates pressure them. Site creator Spencer Thayer, a Chicago activist, described the goal this way in a Twitter message: “So who wants to help start . . . a new website aimed at harassing Democratic Superdelegates?”

Longtime Democratic National Committee member and superdelegate Bob Mulholland wrote a letter to Sanders last week excoriating the candidate for not calling out his supporters for their “bullying” of superdelegates.

Mulholland said he has not received any threats directly but has fielded complaints from other delegates who said they have received harassing emails, Facebook postings and phone calls — including one to a woman at 10:30 p.m. and another that a 12-year-old child picked up.

“Society has been trying to deal with High School bullies and the same Rule should apply to your campaign and your supporters,” wrote Mulholland, who supports Clinton. “Professionally, campaign staff and representatives should be the ones calling delegates.

“A 12 year old child answering the phone at home should not be hearing threats,” he added.

Republicans are in the midst of a fierce delegate fight, too. Front-runner Donald Trump is still trying to win enough nominating contests to take a majority of delegates and claim the nomination outright. Sen. Ted Cruz (Tex.), meanwhile, is executing a state-by-state strategy to install delegates who would choose him on a second ballot if Trump fell short. (GOP rules allow many delegates to switch allegiances if no one wins the nomination in the first round of voting.)

According to some Republican leaders, the intensity of the delegate battle has even sparked death threats against those who have publicly criticized Trump.

On the Democratic side, the Sanders campaign has not publicly denounced Thayer or other supporters, but campaign manager Jeff Weaver said Sanders does not support the lobbying of superdelegates by outside groups or volunteers.

“We certainly don’t condone harassment of anybody,” Weaver said.

Clinton, too, was asked about the issue during a campaign stop this week in New York.

Hillary Clinton leads Bernie Sanders among superdelegates 469 to 38, according to a tally by the Associated Press. (Yana Paskova/For The Washington Post)

“I have heard those reports and had some firsthand accounts of some of the unfortunate behavior that we’ve seen both online and in person,” she said, adding that she is proud that her own campaign has been run “on issues.”

“There seems to be a growing level of anxiety in that campaign, which I hope doesn’t spill over into the way that his supporters treat other people, who have every right to support whomever they choose,” she said. “And where things stand now, I am far ahead of him in the popular vote, considerably ahead in the pledged delegates.”

Sanders is trailing nationally but riding the momentum of a string of recent victories in state primaries and caucuses. Nowhere is his deficit more pronounced than among superdelegates, many of whom are longtime politicians with ties to the Clinton political franchise.

The overall intensity of the courtship of these leaders has not matched that of 2008, when Clinton was running against Barack Obama, many superdelegates say.

Still, Sanders’s campaign has begun to argue that the system is stacked against the candidate and that superdelegates should consider backing Sanders to more accurately reflect the will of voters, who have delivered him victories in seven of the last eight nominating contests.

Outside supporters go further, saying the superdelegate system is designed to protect establishment candidates such as Clinton — and to prevent populist insurgencies. The most frequent targets of that ire are the DNC and Clinton, but Sanders supporters have increasingly advocated direct lobbying of superdelegates.

“It’s time we take our democracy back from the DNC. Together we can find the Superdelegates and hold them accountable to our votes,” reads the “hit list” site’s front page. As for whether his site encourages stalking, Thayer said in a tweet that his list is “no more cyberstalking than an AT&T phonebook or a public registry of elected officials.”

Nonetheless, since its creation last week, the site’s name has changed to Superdelegate List — and its logo from a donkey with arrows through its head to a donkey surrounded by phone cords.

Thayer said in an interview that he made the changes because the name and logo became counterproductive. He said his goal is accountability — and perhaps eventually to do away with a system he said is designed to perpetuate the political power of an elite few.

“Historically, the superdelegates have been able to disenfranchise voters without being held accountable,” he said. “The Internet has changed power relationships between party leaders and their constituents, and those in power have a tendency to interpret challenges to their authority as harassment.

“I’m not sympathetic to that point of view,” he said.

Thayer said the site is entirely separate from the Sanders campaign. Sanders “is irrefutably a better candidate than Hillary Clinton,” he said, but Thayer, an independent, said he did not vote in Illinois’ primary.

Several superdelegates said in interviews that they have been contacted online or by phone by Sanders supporters, who often complain that the system is undemocratic or unfair. None of these delegates said they felt personally threatened, but some found the contacts aggravating.

“People are making a lot of threats and putting a lot of pressure on delegates to switch,” said Ken Martin, the Minnesota Democratic Party chairman and a Clinton superdelegate.

Scott Brennan, a former chairman of the Iowa Democratic Party and a superdelegate supporting Clinton, said he has heard from Sanders supporters but not directly from the campaign. Brennan, who decided to back Clinton shortly before the Iowa caucuses, said he didn’t consider any of the emails or tweets to be out of bounds.

Sanders’s aides said their own effort rests on contacting all superdelegates, especially the roughly 200, out of a total of about 700, who have not publicly committed to either candidate.

A team based in Sanders’s Burlington, Vt., headquarters keeps tabs on those and other superdelegates, checking in regularly and making sure they are available to answer questions, aides said.

The wooing process can also include calls from leading Sanders surrogates, including other superdelegates already committed to the senator from Vermont. When someone is close to committing, either Sanders or his wife, Jane, is tapped to make a phone call and act as “the closer,” Weaver said.

Martin said he has seen no recent defections by Clinton superdelegates, including those who have not made their commitments known publicly. “The only thing we’re seeing is that people are going to continue to wait until their states have weighed in, or until the process is completed in June, to make their intentions known,” he said.

Martin said Clinton has commitments from about 600 superdelegates, a figure well above the roughly 470 listed in most counts kept by the media.

The Associated Press currently gives Clinton 1,280 pledged delegates — those won in nominating contests — and 469 superdelegates, for a total of 1,749. The AP says Sanders has 1,061, including 38 superdelegates.

Each campaign says its number is actually higher, reflecting commitments from supporters who have not been willing to go public yet.

Some superdelegates and other senior Democrats said the Clinton campaign has sought to reassure those who feel under siege — or who are just ready for the long and unexpectedly bitter primary fight to be over.

“We need to get ready for a Hillary versus Republicans election sooner rather than later,” said Rep. Don Beyer (D-Va.), a superdelegate firmly pledged to Clinton.

Beyer said he has not been personally lobbied to change his support either by the Sanders campaign or by others online, but his congressional office said several Sanders supporters have called to urge him to support the senator — and those calls are usually handled by interns.

Beyer added with a laugh: “I made the decision early on not to read my Facebook postings. That way you don’t ever get discouraged or down.”

Weaver said Sanders’s courting of superdelegates will intensify in coming weeks.

Aides said they will have a more credible case to make if Sanders accumulates more pledged delegates than Clinton does by the end of the nominating calendar in June, a long-shot prospect.

They also said that among their targets are superdelegates in states that Sanders has won, particularly those he has won convincingly. Their argument: Their support should mirror that of their constituents.

The outcome Saturday in Wyoming is among the motivations for Sanders supporters. He won the state’s Democratic caucuses but split the 14 pledged delegates. The state also has four superdelegates, all supporting Clinton.

“He got more votes; she got more delegates,” said a frustrated Scott Weiler, 38, an ironworker’s apprentice who brought his family to Sanders’s rally in Albany, N.Y. “That’s bull----. If there were no superdelegates, I think Bernie would win.”

Weiler said he has signed petitions and emails asking Clinton-backing superdelegates, including Gov. Peter Shumlin of Vermont, to consider the support for Sanders in their states — and switch.

Phillip reported from New York. Dan Balz in Washington and David Weigel in Albany and Binghamton, N.Y., contributed to this report.