A spokesman for the Chamber said Thursday that it's a "substantial" ad campaign with a six-figure budget. He declined to disclose a precise figure.
McSally has lagged Democrat Mark Kelly in fundraising all year. She had $5.6 million in the bank at the end of September, about $4 million less than Kelly.
She also faces at least one challenger for the GOP nomination.
The Chamber's ad highlights McSally's support for updating the North American Free Trade Agreement. The ad says no one has worked harder to pass the trade deal, known as the U.S.-Mexico-Canada Agreement.
An independent group has attacked McSally in ads focused on health care.