Backers of Democratic presidential contender Bernie Sanders have launched a campaign to win over some of his rival Hillary Clinton's most prized supporters - the superdelegates that can make the difference in a tight race for the party's nomination.

But some emails, phone messages, and petitions sent by the Sanders boosters have backfired, upsetting superdelegates with their aggressive tone and leading many to dig in their heels for Clinton, according to interviews conducted by Reuters.

The drive to flip Clinton's super delegates has not been sanctioned by Sanders' campaign, his spokesman Michael Briggs said.

"Bernie's campaign is focused on reaching out to all voters and earning delegates at primaries and caucuses," he said in a statement, stressing that the Sanders campaign was not coordinating with supporters to contact superdelegates.

RELATED: Democratic Party slams GOP candidates on climate change

However, the unofficial push could complicate the U.S. Senator from Vermont's efforts to woo the critical bloc in the coming months.

The effort has at times taken an angry tone, some of the messages reviewed by Reuters showed, reflecting the anti-establishment tinge of the 2016 presidential race where many voters are unhappy with Washington insiders.

Some 85 percent of the 4,763 delegate votes to the Democratic National Convention that will decide who will face a Republican rival in the November election are determined by the results of states’ nominating contests. But the remaining 15 percent are held by superdelegates, who get to vote however they like - meaning they could hold the key to a tight contest.

Superdelegates are made up of party leaders and elected Senators, members of Congress, and governors. The Democratic party adopted the system in the early 1980s as a way of giving party leaders more control over the nominating process, though they have yet to play a decisive role in a nomination.

"The idea there is that you’ve got people who have a long view ... who have, arguably, the best interests of the party at heart," said Terri Fine, a professor of political science at the University of Central Florida.

At least one superdelegate who has backed Sanders, however, finds the system that can overturn the will of regular voters flawed. “I’m a superdelegate to fight to end superdelegates,” said Larry Cohen, a former president of the Communications Workers of America union who volunteers full-time for the Sanders campaign.

Among super delegates that have tipped their hand so far, Clinton holds 453 to 20 for Sanders, according to the New York Times delegate tracker. Among regular delegates, won through nominating contests, Clinton leads by 91 to 65 after the South Carolina primary. (Graphic: http://tmsnrt.rs/1QHZCSj )

The Republican party does reserve slots for members of its national leadership, but they do not have such influence as Democratic superdelegates.

PRESSURE TACTICS

Interviews with 10 of the 505 super delegates supporting Clinton Reuters has reached show that nine of them have been approached by people purporting to back Sanders, and nearly all were displeased by the tone of the outreach. 

Isabel Framer of Ohio, a superdelegate for Clinton, for example, got a voice mail last week urging her to vote for Sanders “in accordance with the will of the people.”

On the voice mail, heard by Reuters, the anonymous male caller says: “I think it’s crap that you get to vote whichever way you want... I’ll be watching your vote.”

“I’m not easily frightened,” Framer told Reuters. “I’m not going to change a vote over threats.”

RELATED: These are the top words used by presidential candidates during the debates

Akilah Ensley, a North Carolina superdelegate, said she started hearing more often from Sanders supporters after her name appeared on a Wikipedia list noting her support for Clinton. "Some of them were nice, and some were rather abrasive," she said, adding "attacking my decisions is probably not the best way” to change her mind.

Luis Heredia, an Arizona superdelegate for Clinton, said he has received over 30 phone calls, emails and instant messages from Sanders supporters. “The majority of them are more angry, and the tone is more demanding,” Heredia said.

Lacy Johnson, an Indiana superdelegate backing Clinton, meanwhile, said he had received a mix of messages, including one that he said threatened: “we will make you pay.”

Andres Ramirez, a political consultant in Las Vegas, Nevada, and a superdelegate supporting Clinton, said in the past campaigns would typically try to soft-sell their candidates rather than use pressure tactics.

"The way this has gone down, in my experience, has never happened before," said.

"TASK FORCE ARMY"

While it is unclear who is directing some of the calls and emails to Clinton's superdelegates, Seattle resident Justin Renquist has pushed for some of the outreach, although he stressed that such contacts needed to be civil. Renquist calls himself part of the “Superdelegate Task Force Army” that rallies Sanders supporters to reach superdelegates via Facebook, Twitter and other means.

“There are 3,000 in my group who have been slamming these guys as politely and nicely as we can...basically saying, look this system is undemocratic,” Renquist said.

On Friday, the group issued a statement saying it did not support, condone or engage in any communication with superdelegages that was "vulgar, threatening, harassing or intimidating."

Other Sanders fans, meanwhile, have gathered more than 300,000 signatures under at least two petitions on MoveOn.org, asking superdelegates to vote in line with the results of the primaries and caucuses.

A third petition, on the Democracy for America site, also asks superdelegates to follow state nominating contests. It was started by Robert Reich, who was a secretary of labor under Bill Clinton and who has endorsed Sanders.

The Sanders campaign plans its own effort to win over superdelegates, possibly in April.

"That’s when we’ll make a much stronger approach to superdelegates,” said Tad Devine, a senior adviser to the Sanders campaign.

Clinton's campaign, meanwhile, started officially courting superdelegates months ago.

“We are proud to have the support of these Democratic party leaders, who have been in the trenches,” said Jesse Ferguson, a spokesman for the Clinton campaign.