David Brock, speaking at the Clinton School of Public Service in Little Rock, in 2014. (Danny Johnston/AP)

David Brock is sorry. The founder of Media Matters and a pair of pro-Clinton super PACs battled hard — really hard — against Sen. Bernie Sanders during the Democratic presidential primary. Now he wants to be Sanders's ally.

“I'm with you in the fight ahead,” Brock wrote Tuesday in an open letter to Sanders (I-Vt.), posted on Medium.

“At times during the Democratic primary, I was criticized for being too aggressive in my support for Secretary Clinton  — and rightly so,” Brock added. “Looking back, I recognize that there were a few moments when my drive to put Hillary in the White House led me to take too stiff a jab. I own up to that, I regret it, and I apologize to you and your supporters for it.”

Perhaps Brock is thinking of the time he said that “it seems black lives don't matter much to Bernie Sanders,” or the time one of his groups, Correct the Record, compared Sanders to polarizing British politician Jeremy Corbyn, or the multiple times that another of Brock's groups, the American Democracy Legal Fund, accused the senator from Vermont of Federal Election Commission violations.

Sanders, naturally, was not a David Brock fan during the campaign. He referred to Brock as “scum of the Earth” in an interview with Time magazine last May. Sanders's office did not immediately respond to a Fix inquiry about whether the senator accepts Brock's apology or wants his onetime nemesis to be with him in “the fight ahead.”

Brock told me that he hopes to win over Sanders by advocating for causes they both care about, such as combating climate change and the influence of big money in politics.

“I don't expect anyone to accept one Medium post at face value, especially in a cynical town like Washington,” Brock said. “I'll earn the trust over time, as we work together to resist the Trump administration and to preserve and protect progressive accomplishments.”

A mutual embrace between Sanders and Brock would be more complicated than setting aside hurt feelings, however. Sanders would risk looking like a hypocrite after he criticized Hillary Clinton for aligning herself with Brock, whom he described in the Time interview as someone “who has spent his life as an attack dog, who has gone about trying to destroy political opponents.”

“They have been attacked in that way for decades — the Clintons have — by people like David Brock, before he presumably saw the light, who lied about the Clintons,” Sanders said. “And their response is, 'Look, that's the world we live in. That's what you gotta do.' I understand that. I don't think that's what you gotta do. I don't think you hire scum of the Earth to be on your team just because the other side does it. You gotta play in a little bit different way.”

It sure sounded like Sanders was questioning the authenticity of Brock's progressive conversion when he said that the former conservative journalist “presumably saw the light.” Brock, a thorn in the Clintons' side during his time at the American Spectator in the early 1990s, famously renounced his conservatism, apologized for self-described lies and distortions, and became a leading Clinton defender.

I asked Brock why Sanders and his supporters should believe that he is sincere now and not just doing what is politically expedient — changing teams again because Sanders seems like a better investment than the vanquished Hillary Clinton.

“I didn't view the Clintons as a 'good investment,' " Brock replied. “I sincerely believed that Hillary would have been one of the great presidents. I still hold that view, and as everyone watching knows, I knocked myself out to help her achieve that goal, not for her or me but for the country. I expect to continue to wholeheartedly support their future venues. My sincere words to Sen. Sanders are about an issue between us; they are not at Hillary's expense.”