WASHINGTON – Some of Bernie Sanders’ staunchest supporters are frustrated he’s not going after Hillary Clinton more forcefully, especially as she continues to expand her delegate cache.

“I think it’s hurt him a little bit. I think Bernie is a gentleman,” said Michigan state Sen. Bert Johnson, who has been trying to get Sanders to toughen up on the campaign trail.

“Sometimes you have to take those gloves off.”

Sanders has given Clinton plenty of passes.

At the first debate, the Vermont senator famously said he doesn’t care about her “damn emails,” Clinton’s biggest potential liability.

In January, Sanders said Clinton should not be judged by the “totally disgraceful” affair of her husband.

Just Monday night, Sanders punted a question from Fox News’ Bret Baier about whether Clinton can be trusted.

It’s a tricky tightrope for Sanders, who has sworn off negative campaigning and pledged an issues-focused campaign.

But a skillful Clinton has managed to get under his skin.

She accused him of an “artful smear” during a February debate for insinuating she’s bought by Wall Street.

She dropped a bomb on him during the Flint debate Sunday for accusing him of being against the vital auto bailout, forcing Sanders to launch radio ads in Michigan explaining he really did vote

for the auto rescue, just not the Wall Street bailout funds that financed.

Supporters say Sanders needs to stop being Mr. Nice Guy.

“Senator Sanders has been too easy on Senator Clinton,” said Flint resident Alex Richman, 27, a night stock manager at a Michigan grocery store.

“In the debate alone, he let her speak over him. I feel that she is a lot more honest and trustworthy than any of the Republican Party, but unfortunately that isn’t saying much.”

Richman has $50,000 in college debt and says he trusts Sanders will fulfill his campaign pledge to boost the economy.

“He is the only candidate that when speaking, I believe everything he says he will do,” Richman told The Post.

Johnson, the Michigan state senator, says handling Clinton with kid gloves won’t do her any favors if she becomes the nominee.

“Go right at the heart of history, which is the idea of her trustworthiness,” Johnson said. “He wouldn’t be the only one questioning it. There’s an entire American electorate questioning that.”

But Rep. Alan Grayson (D-Fla.) believes Sanders’ commitment to stay clear of Clinton’s private email server shows his discipline and his commitment to creating progress in America.

“He’s never run a negative campaign ad in his entire life, ever,” Grayson told The Post. “He is extremely disciplined and tenacious about talking about issues and how he can change the lives of ordinary Americans for the better.”