When Cory Booker announced his presidential bid in February he pledged to wage a campaign around unifying the country, calling for “radical love” of all people.

But as he continues to slip in polls and fundraising momentum, Booker’s love for his competitors, radical or otherwise, has frequently slipped into passive-aggressive digs, even taking time to punch down at the least competitive contenders.

“It struck me as ironic because Booker is running as this kumbaya candidate,” one senior official on a rival campaign said, describing an indirect shot fired by the senator at their candidate. “To be running on such a positive platform but calling people out without actually calling them out, it seemed kind of desperate.”

Sometimes the shots are direct. When Sen. Bernie Sanders (I-VT) called for restoring voting rights to convicted felons—including the Boston Marathon bomber—still serving prison time, Booker slammed him for taking the focus off “black and brown people and low-income people” who are languishing in prison.

Others have been more discreet. After Sen. Kamala Harris (D-CA) copped to smoking marijuana in her youth on a radio show in a light-hearted exchange, a month later, Booker took a swipe at “senators” who joke about having used the substance that has led to the incarceration of thousands of people.

But it’s not just the top tier that has drawn Booker’s fire.

In an email sent last month to supporters, Booker attacked former Rep. John Delaney, a Maryland businessman, for infusing his campaign with millions from his personal fortune. Delaney, despite campaigning for the nomination for several years, still polls in the low single digits.

Still, on April 15, Booker’s campaign sent out a note that said: “This weekend, we found out that one of the other Democrats in this race has given over $11 million of his own money to his campaign,” which was italicized in the email for emphasis. “Self-funding is something Cory just can’t and would never do.”

Asked about the jab on the campaign trail, Booker later told reporters, “I’m not even sure what you’re talking about, because again we are not taking swipes at other candidates.”

But Delaney’s team saw it differently. “We were surprised,” a senior official in his campaign told The Daily Beast. “Booker has such high name ID, it seemed like an amateur move to go after us.”

Sabrina Singh, Booker’s national press secretary, disputed that he had gone negative on his rivals.

“Check the context of Cory’s comments before jumping to conclusions,” Singh said. “I know some people in politics are driven to parse words and create controversy in even the most respectful exchanges, but the fact is Cory’s gone out of his way to lift up and celebrate his fellow Democratic candidates and run a campaign centered on bringing Americans together to beat Donald Trump and take on the toughest challenges we face.”

Booker, along with other top-tier 2020 contenders, signed a pledge early in the cycle by the progressive movement Indivisible to have a “constructive primary.” In it, Booker and others pledged to “remain grounded in our shared values, even if we support different candidates.” Signers also agreed to rally behind the party’s nominee and to work to defeat President Trump.

Taking stabs at rival campaigns in an competitive cycle is not uncommon. And in a field with nearly two dozen contenders, candidates have spread their attacks across multiple targets. But Booker’s decision to wade into negative territory, albeit subtly, is significant. Most of his public brand has been built with an extra emphasis on civility and love.

While Booker has taken jabs at lesser-known rivals, the “Street Fight” senator has also punched above his weight.

During an interview on the radio show The Breakfast Club in February, Harris joked that she previously smoked marijuana. “I have,” Harris said. “And I inhaled—I did inhale. It was a long time ago. But, yes.”

It was a moment Booker couldn’t ignore. In a cable news interview the following month, he addressed decriminalizing marijuana, one of his signature campaign issues, while adding a critique of “senators” who joke about having used the substance.

“We have presidential candidates and congresspeople and senators that now talk about their marijuana use almost as if it’s funny,” he said. “But meanwhile, in 2017, we had more arrests for marijuana possession in this country than all the violent crime arrests combined.”

Booker has also lashed out at Sen. Elizabeth Warren (D-MA). When asked in an interview with ABC’s This Week if he thinks Facebook should be broken up by the government, a plan Warren proposed along with other sweeping tech regulations, he said he disagreed with the idea that “a president should be running around pointing at companies and saying ‘breaking them up’ without any kind of process.”

He also seized on an opportunity to distance himself from Sanders, who called for restoring voting rights to convicted felons still serving prison time. During a televised town-hall forum in New Hampshire, Sanders said he would hypothetically support restoring voting rights for Dzhokhar Tsarnaev, the man who carried out the Boston Marathon bombing in neighboring Massachusetts.

In an interview a few days later on PBS Newshour, Booker dismissed Sanders’ answer as a distraction.

“If Bernie Sanders wants to get involved in a conversation about whether Dylann Roof and the marathon bomber should have the right to vote, my focus is liberating black and brown people and low-income people from prison,” he said.

On the road Booker has made it clear that his “love and unity” campaign does not mean he’s weak and that he was unafraid of “tough politics.”

During a CNN town hall in March, he was asked by an attendee if a positive message could work in the current political climate, particularly against Trump.

“I am so grateful for this question, because I do hear it from time to time, even from some of my friends,” Booker conceded, before flexing his figurative political muscles.

“I've gone up against titans, bullies, through my New Jersey politics. In fact, I don’t think anybody in this race has been through the kind of tough politics I’ve been through,” he said. “There’s even a documentary about it called Street Fight.”

“But we win this election not by showing the worst of who we are but by the best of who we are,” he said. “You see, Donald Trump wants us to fight him on his terms. To me, that is not only a recipe for losing the election, it’s a recipe for losing the ability to move this country forward.”