Is Joe Biden to blame for vaccine hesitancy because he said he did not trust Donald Trump? “Not that we’ve seen in the data,” replied Jen Psaki, the White House press secretary. That was the feint. Then came the thrust for the jugular.

“I would note that at the time, just for context, the former president was also suggesting people inject versions of poison into their veins to cure Covid.”

For questioner Peter Doocy, White House reporter for the conservative Fox News channel, there was no coming back from that verbal roundhouse kick from Biden’s top media spokesperson. Until next time of course.

For four years the White House must have felt like home turf for Fox News. President Donald Trump was a regular interviewee on the network while describing other media as the enemy of the people. His press secretaries Sean Spicer, Sarah Sanders and Kayleigh McEnany duly took a combative approach to the briefing room, tossing out verbal grenades that usually failed to explode.

Now the shoe is on the other foot and Fox News finds itself as a leading voice of dissent at the daily press briefing, asking some of the bluntest questions as it tries to pick holes in the Biden administration.

The duels between Doocy, son of Fox & Friends co-host Steve Doocy, and Psaki, an alumnus of the Barack Obama administration, hardly rank alongside Spicer’s televised meltdowns, but do offer insights into rightwing critiques of Biden and his strategy for neutralizing them.

Doocy, 34, comes to each briefing armed with questions carefully crafted to elicit, some would argue, a “gotcha!” moment or at least a viral clip, even if that clip consists merely of “White House denies” whatever wild charge is thrown at it.

Psaki, 42, for her part, walks a fine line between hearing Doocy out with patience and courtesy – even a hint of loss of temper would be disastrous optics – while putting him in his place with a cutting phrase, known on social media as a “#PsakiBomb”.

Whereas most questions at the briefing will produce a paragraph or two at best, Doocy seems to think in terms of headlines, albeit sometimes disingenuous ones. On 16 July, for example, he went straight for the bullseye with: “For how long has the administration been spying on people’s Facebook profiles, looking for vaccine information?”

It was an absurd premise, based on officials’ statements that vaccine disinformation on Facebook can be traced to about a dozen people, ignoring the fact this figure comes from a report by the Center for Countering Digital Hate, not the administration’s own surveillance.

Psaki replied crisply: “That was quite a loaded and inaccurate question. This is publicly open information, people sharing information online, just as you are all reporting information on your news stations.”

On 20 July, Doocy noted that at least six Democrats who fled Texas to thwart a voter suppression bill in the state had come to the US capital and tested positive for coronavirus despite being vaccinated. “Is there any concern that this trip that was intended to advocate for voting rights is now a super-spreader event in Washington?” he asked.

First, Psaki swerved past the obvious trap: “Well, I would say that’s not a characterization we’re making from here. We certainly understand there will be breakthrough cases. Even vaccines that are incredibly effective are not foolproof. They’re not 100% effective. We’ve seen that.”

Then she tried to spin the episode into something more positive: “Here’s the good news, though, we know that these individuals, I think, if I’m correct, have been vaccinated. It means that it protects from death; it protects from serious illness; it protects, for the most part, from hospitalization. So, that is a good sign.”

On the pandemic again a week later, with masks returning to the briefing room, Doocy voiced the frustration of many on his network by asking that, if vaccines work, “why do people who have the vaccine need to now wear masks the same as people who have not had it?”

Psaki appeared irritated and defaulted to an answer that not everyone felt satisfactory: “Because the public health leaders in our administration have made the determination, based on data, that that is a way to make sure they’re protected, their loved ones are protected. That is an extra step, given the transmissibility of the virus, that they’re advising people to take.”

Last week Doocy seized with relish on Democratic congresswoman Cori Bush hiring private security while calling for the defunding of police. Psaki replied calmly, “Well, I think we shouldn’t lose the forest through the trees here,” pointed out that Bush has received death threats, then added, “I will say that the president has been crystal clear that he opposes defunding the police.”

The exchanges add a jolt of electricity to what can seem incremental questions and repetitive answers on these long, humid and soporific summer afternoons in Washington. Empty seats, once unthinkable in the Trump era, have begun to reappear in the briefing room. Doocy’s aggressive approach and willingness to interrupt Psaki keep her on her toes.

His presence also gives the White House access to a Fox News audience that it might otherwise lack, and allows the administration to present itself as willing to entertain diverse opinions rather than groupthink. So when last month Doocy returned from a brief spell away and Psaki provoked laughter by telling him, “Welcome back!”, she was perhaps being both sarcastic and sincere.