Barack Obama might not be on the Democratic ballot, but he's on the airwaves

As the 2020 Democratic primary season enters an ever-more aggressive phase, a surprise guest is finding himself a star of many candidates' ad campaigns: former President Barack Obama.

While Obama has mostly stayed on the sidelines during the primary contest, many of the candidates are trying to push the messages that they're closely tied to him, plastering their latest ads with the face and voice of the former president.

"This is an extraordinary man with an extraordinary career in public service," Obama says about his former vice president in Joe Biden's latest television ad launched on the eve of the New Hampshire primary.

From start to finish, the ad features Obama's voice in a clip from January 2017, with the former president listing his then-vice president's record on revitalizing middle class America, combating gun violence, supporting affordable college and fighting violence against women. The ad ends with Obama saying: "All of this makes him the finest vice president we have ever seen. The best part is, he's nowhere close to finished."

The latest Biden ad comes on the heels of a weekend-long squabble between Biden and former South Bend, Indiana, Mayor Pete Buttigieg, after Biden said that Buttigieg is "not a Barack Obama" on the campaign trail and the 38-year-old shot back, "Neither is he," on ABC's "This Week."

And yet many of the contenders are still dropping his name, touting their connections to him, and blanketing the airwaves with ads featuring Obama, as New Hampshire voters prepare to cast their votes on Tuesday.

The latest ad from Sen. Elizabeth Warren, D-Mass., that began airing last week is narrated by Obama's voice from more than a decade ago, using his remarks on the Consumer Financial Protection Bureau from the Rose Garden in 2010.

"She's a janitor's daughter who has become one of the country's fiercest advocates for the middle class," Obama's voice says over shots of Warren through the years -- including her standing with Obama. "She came up with an idea for a new, independent agency standing up for consumers and middle class families. She's done it while facing some very tough opposition. Fortunately, she's very tough."

"Elizabeth understands what I strongly believe: that a strong, growing economy begins with a strong and thriving middle class," Obama's voice continues.

Former New York Mayor Michael Bloomberg released an ad last week also highlighting his connections to Obama, featuring Obama's remarks from 2013, praising Bloomberg as a "leader" and touting their efforts supporting gun safety over the years.

In a tweet accompanying the ad, Bloomberg said, "President Barack Obama is someone I was proud to call Commander-in-Chief, and someone I was honored to work with to get big things done. I miss that feeling."

In almost all of his recent ads, entrepreneur Andrew Yang has touted the fact that Obama named him the "champion of change," and Sen. Bernie Sanders, I-Vt., too has often featured Obama remarks on the importance of health care in his campaign ads.

And while Buttigieg's recent ads don't feature Obama, his latest ad blitz to follow the New Hampshire primary revolves around rust belt states, focusing on counties that flipped from Obama to Donald Trump in 2016.

But the Biden campaign continues to claim the mantle of the Obama legacy.

"I think voters know that Joe Biden was Barack Obama's vice president," Biden's deputy campaign manager Kate Bedingfield said on Monday. "If other candidates are trying to lay a claim to a better partnership with or a bigger piece of the Barack Obama legacy, I wish them good luck with that."

ABC News' John Verhovek, Molly Nagle and Sasha Pezenik contributed to this report.