It started during last week’s Republican convention, a four-day spectacle of red-meat, hard-core conservative politics in Cleveland where delegates openly clashed over the candidacy of the billionaire Donald J. Trump. On the final night, Mr. Trump delivered a dystopian address that presented America as a fearful place, surrounded by darkness and besieged by enemies. The atmosphere in the hall was strange and electrifying, punctuated by chants of “U-S-A!”

This is my first time covering a campaign in America, and a far cry from those I remember as a child in Ireland, where my father was active in local politics for Fianna Fail, then the governing party. At home, political reporting involved a tour of what was sardonically known as the “chicken-and-chips circuit,” named for the dominant catering choice at campaign events across the country.

In Cleveland, news organizations and tech firms rented out restaurants near the convention arena, providing open bars and free food to journalists, political operatives and corporate types. The Washington Post, CNN and Twitter had their own hospitality setups at restaurants on East Fourth Street, a pedestrian walkway thronged by people and hawkers selling campaign paraphernalia: T-shirts that read “Hot Chicks for Trump,” and pins denigrating Mrs. Clinton.

The media circus also offered an irresistible opportunity for other Americans, of every persuasion, to push their own causes. In front of the NBC studio, which overlooked the street, people waving signs jostled for space, trying to edge into the shot during a live panel discussion. One evangelist held a sign that read “REPENT: Fear him who has the power to cast you into hell” while another yelled into a bullhorn. A mustachioed young man was promoting a T-shirt company. And a diminutive woman, standing on her tiptoes, held a sign warning of the dangers of something call gadolinium.

“The world’s watching,” she said. “I can’t allow another person to die.”

It felt like more like street theater than politics. But the more salient issues of the American campaign were never far. The shooting of three police officers in Baton Rouge, La., on the eve of the Republican convention highlighted the heady mix of guns, race and policing that has played prominently in the presidential race.

In Philadelphia, America’s wider malaise is playing big. Sanders supporters are overwhelmingly young and, like their Trump counterparts, angry at the traditional establishment of their party. The weekend email dump by WikiLeaks seemed to confirm their suspicions that the party establishment was allied against them.