New polling shows Vice President Harris opening up a modest lead over President Trump as she and her new vice presidential pick, Governor Walz, embark on a swing state tour.

A new survey from YouGov and the Economist found that Ms. Harris leads Trump in a multi-candidate field by 45 percent to 43 percent, maintaining her margin since the pollster’s last survey in late July.

The pollster also found that voters under age 30 have swung dramatically in Ms. Harris’s favor since mid-July. In their July 16 survey, a potential Harris ticket led Trump by 13 points with younger voters; now, she leads Trump by 31 points among that demographic.

Ms. Harris’s lead was within the survey’s margin of error, which was plus or minus 3 points. It is not the only survey in recent days to show Ms. Harris enjoying a slight edge over Trump in the polls.

In the FiveThirtyEight polling average, Ms. Harris leads Trump by 45.2 percent to 43.4 percent. In the RealClearPolitics polling average, Ms. Harris now leads Trump by 0.5 points.

The polling suggests that, nationally, the race remains close after weeks of turmoil that saw President Biden drop out, an assassination attempt against Trump, and Ms. Harris becoming the presumptive nominee.

It also sets the stage for the next 12 days, which will see Ms. Harris and Mr. Walz tour the country ahead of the Democratic National Convention. Trump’s running mate, Senator Vance, is also planning to follow the campaign through the swing states, holding competing events.

In those swing states, the race looks similarly close to the national stage, according to a survey from Redfield Wilton and the Telegraph released Tuesday.

The survey found that of the six main swing states, Trump leads in Pennsylvania by 2 points, Michigan by 1 point, and in Georgia by 2 points. Ms. Harris leads Trump by 1 point in Arizona and the two are tied in Wisconsin and Nevada.

In Wisconsin specifically, a new survey from one of the state’s best pollsters, Marquette University Law School, found that Ms. Harris enjoys a 1-point advantage in the state, leading Trump by 50 percent to 49 percent.

Sabato’s Crystal Ball at the University of Virginia’s Center for Politics also released new ratings reflecting the race in these states, with all six swing states being rated as “toss ups.” Previously, Sabato’s Crystal Ball had given Republicans a small advantage in Georgia.