The request for a bid for the Democratic National Convention had been sitting unanswered in the office of Mayor Lori Lightfoot of Chicago for two to three weeks in the summer of 2021 when Anne Caprara, the chief of staff to Gov. JB Pritzker of Illinois, burst into the governor’s office with a Politico article saying Chicago was being courted.

“Why wouldn’t Chicago do this?” Mr. Pritzker asked, according to his senior aides.

Three eventful years later, Chicago has a new mayor, the Democratic Party has a new standard-bearer, and the city is completing final preparations for the convention. Through it all, Mr. Pritzker has been the constant, the anchor to which the city’s effort has been tied.

In the days before the curtain is raised, the governor has been out and about in his hometown — in a tavern across from Wrigley Field chatting with the MSNBC host Jen Psaki, at Wrigley talking to convention volunteers, at a thank-you luncheon at the International Brotherhood of Electrical Workers union hall on Saturday. On Sunday, he is set to appear at a Chicago taping of “The Late Show With Stephen Colbert.”

In short, the city might be the mayor’s, but the party this week will be the governor’s.

“I think his role can’t be overstated,” said State Representative Kam Buckner, an ally in the legislature and a rising star in Chicago politics. “The world is watching, and Governor Pritzker is acutely aware of that.”

The political stakes of the convention changed for Mr. Pritzker after President Biden’s decision to drop out of the race and the elevation of Vice President Kamala Harris to lead the ticket.