If Bernie Sanders doesn’t receive a Father’s Day card on Sunday, the reason might not be hard to figure out.

The U.S. senator from Vermont has declined to endorse his son Levi Sanders, 49, a candidate for a U.S. House seat in New Hampshire, according to reports.

“He’s on his own,” the elder Sanders told CNN’s Chris Cuomo on Thursday night, adding that he doesn’t like “dynastic politics.”

"He's on his own." — Bernie Sanders, referring to son Levi's run for a congressional seat

Some say they are not surprised, because the elder Sanders has a long history of being stingy with political endorsements.

In the mid-1980s, for example, Sanders declined to do more to aid what has evolved into Vermont's Progressive Party. Then last winter he declined to help his stepdaughter, Carina Driscoll, run for the mayor's office in Burlington, Vt., which Sanders himself once held.

“This is a fancy dance that Bernie has done, but it's not recommended for anyone else," said Garrison Nelson, a University of Vermont political science professor emeritus. “This is a testament to Bernie’s uniqueness as a political actor.”

“This is a testament to Bernie’s uniqueness as a political actor.” — Garrison Nelson, political science professor emeritus, University of Vermont

For his part, Levi Sanders -- a legal services analyst who is one of 11 seeking the Democratic nomination to fill the seat held by retiring U.S. Rep. Carol Shea-Porter -- has joked that rather than being Bernie Sanders' son, he is the son of the fourth cousin of Larry David, the co-creator of “Seinfeld” who has portrayed Bernie Sanders on “Saturday Night Live.”

But since the father-son political relationship has drawn recent attention, Levi Sanders has gone quiet on the matter.

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.

His campaign said in a statement Saturday that he also believes in equal pay for women and "a minimum wage allowing people to work 40 hours a week without living in poverty."

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

Many have said Levi Sanders 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.

The Associated Press contributed to this story.