Democrats managed to be in two places at once on Tuesday night, holding a ceremonial roll-call vote at their Chicago convention to celebrate Vice President Kamala Harris as their party’s nominee, while she herself rallied supporters roughly 80 miles north in Milwaukee.

Ms. Harris’s choice to appear in Milwaukee, the largest city in a crucial battleground state, was intentional and pointed: She stood onstage in the same arena where former President Donald J. Trump accepted the Republican nomination last month.

For much of the evening in Milwaukee, the Harris campaign used the arena’s Jumbotron to pipe in the events taking place in Chicago at the Democratic National Convention. But after Gov. Gavin Newsom of California announced his state’s votes for Ms. Harris, ending the roll call of 57 states and territories, Ms. Harris and her running mate, Gov. Tim Walz of Minnesota, marched onto the stage in Milwaukee.

For a moment, she was speaking to two packed arenas at the same time, celebrating the roll-call vote in front of tens of thousands of people, with millions more watching on screens. The two-city rally represented a significant flexing of Democratic muscle with the presidential election just 76 days away.

“We are so honored to be your nominees,” Ms. Harris said. “Together, we will chart a new way forward.”

The Milwaukee rally was just the latest event at which the Harris campaign filled a major arena with Democrats. For more than a year, they had largely stayed away from events featuring President Biden, who drew crowds only in the low thousands.