Vice President Kamala Harris and former President Donald J. Trump are in an even tighter race in the battlegrounds of Michigan and Wisconsin than just seven weeks ago, according to new polling from The New York Times and Siena College.

Ms. Harris’s advantage from early August has been chiseled away slightly by Mr. Trump’s enduring strength on economic issues, the polls found, a potentially troubling development for the vice president given that the economy remains the most important issue driving voters.

With less than 40 days until Election Day, the race is essentially tied in Michigan, with Ms. Harris receiving 48 percent support among likely voters and Mr. Trump garnering 47 percent — well within the poll’s margin of error. In Wisconsin, a state where polls have a history of overstating support for Democrats, Ms. Harris holds 49 percent to Mr. Trump’s 47 percent.