WASHINGTON—Political headwinds are building against a marketing alliance between Verizon Wireless, the nation's largest wireless operator, and cable giant Comcast Corp., amid worries that cooperation between the two rivals could blunt competition in the telecommunications market and lead to higher prices.

The resistance comes as the two companies are extending their partnership to a third test market—the San Francisco Bay area. Under their deal, Verizon Wireless will promote Comcast's Internet, phone and television service...