An endorsement by Bernie Sanders is one of the most coveted gifts in Democratic politics. But the Vermont senator and liberal darling is notoriously reluctant to back many politicians, including, it turns out, his own son.

Sanders has come under scrutiny in recent weeks for not publicly endorsing his son, Levi, in his race for a New Hampshire congressional seat. But people who know him say Sanders, a potential top-tier contender for the 2020 Democratic presidential nominee, has always been stingy with endorsements and that parsimoniousness is extending to his own family.

Going back to the mid-1980s, when Sanders declined to do more to help what has evolved into Vermont's Progressive Party, to last winter when Sanders declined to help his step-daughter run for the mayor's office he once held, Sanders has frustrated many by his refusal to help.

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But that took on additional weight when it was noticed that, despite his crisscrossing the country endorsing liberal candidates, Sanders has declined to get actively involved in his 49-year-old son's race. Levi Sanders, a legal services analyst, is now one of 11 seeking the Democratic nomination to fill the seat held by retiring U.S. Rep. Carol Shea-Porter.

It's hard to square the non-endorsement, especially for the political movement he helped create in Vermont, said University of Vermont political science professor emeritus Garrison Nelson, who has watched Sanders' political career for decades.

"This is a fancy dance that Bernie has done, but it's not recommended for anyone else," said Nelson. "This is a testament to Bernie's uniqueness as a political actor."

When asked about his son's run for Congress, Sanders has said he doesn't like "dynastic politics," and although he's proud of the work Levi Sanders has done, "he's on his own."

Sanders' 2018 senate re-election campaign declined to answer further questions about the issue.

For his part, Levi Sanders has joked that rather than being Bernie Sanders' son, he is the son of the fourth cousin of Larry David, creator of "Seinfeld" and Bernie Sanders' portrayer on "SNL." But since attention was focused on his political relationship with his father, Levi Sanders has also gone quiet.

Levi Sanders is portraying himself as a progressive campaigning for tuition-free college, health care for all and sensible gun legislation. The first two issues were central to his father's latest presidential campaign.

But his campaign has struggled to gain traction, partly due to a crowded field for the Sept. 11 primary. Levi Sanders has only raised about $11,500 through March, according to the latest campaign finance reports.

Many have said he has done little to build support among the grassroots progressives who supported his father. Several analysts questioned whether an endorsement from his father would make all that much difference.

"Levi's candidacy has been a bit of curiosity," said Dean Spiliotes, a longtime New Hampshire political observer and a civic scholar at Southern New Hampshire University.

There is precedent for Bernie Sanders not doing more to help relatives. Last winter, Bernie Sanders' stepdaughter, Carina Driscoll, ran unsuccessfully for mayor of Burlington, the office Sanders used to launch his political career.

When Driscoll announced her candidacy for mayor, Sanders said he and his wife wanted to be respectful of her desire to run on her own.

Vermont's Progressive Party Lt. Gov. David Zuckerman said Sanders has always wanted people to step up on their own.

"I think the Progressive Party is much stronger for it because we have built an organization that is not dependent on any one person," he said.

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AP Reporter Michael Casey in Concord, New Hampshire, contributed to this report.