When Vice President Kamala Harris rolled out her economic agenda, she went to Pittsburgh. When she unveiled her running mate, she went to Philadelphia. And when she had to pick a place for Barack Obama’s first fall rally this Thursday, it was back to Pittsburgh.

Former President Donald J. Trump has earmarked the greatest share of his advertising budget for Pennsylvania and has held more rallies in the state than in any other battleground since Ms. Harris joined the race — including two on Wednesday and three in the last week.

Welcome to the United States of Pennsylvania.

There may be seven main battlegrounds in the race for the White House in 2024, all of which could prove crucial. But Pennsylvania stands apart as the state that top strategists for both Ms. Harris and Mr. Trump have circled as the likeliest to tip the election.