WASHINGTON — In his first, rocky week as President Trump’s press secretary, Sean M. Spicer was scolded by his boss, pilloried as a liar, hammered by journalists, mocked by Stephen Colbert, taunted by the flash-frozen ice cream brand Dippin’ Dots and held up as the poster child for an administration that can play fast and loose with the facts.

No wonder he was looking for his flak jacket.

“Is this bulletproof?” Mr. Spicer asked one afternoon last week, peering into a closet in his sparse West Wing office as he hunted for the combat vest that, by cheeky tradition, is passed down from one presidential spokesman to the next.

Until recently, Mr. Spicer was the public voice and chief strategist of the Republican National Committee, the epitome of establishment Washington. Now he is the face of an administration bent on upending the status quo and waging war on the news media, surprising colleagues here with how comfortably he has embraced Mr. Trump’s ire toward the press.

The day after the inauguration, he marched into the White House briefing room on Mr. Trump’s orders and lambasted stunned reporters as “dishonest” while claiming, against available evidence, that the inauguration had been the most attended in history. (He later said his count included viewers watching online.) The ironic hashtag #spicerfacts was soon trending online.