“Health care costs will skyrocket,” the advertisements warn, urging Americans to “Tell Congress” to act to continue the tax credits as the video shifts to a view of the Capitol building. Health insurers, long concerned a spike in insurance premiums triggered by the subsidies’ expiration will drive away millions of customers, is seizing the moment as their best shot at heightening public attention to their message. The advertising campaign, running across Hulu, Google and Facebook, kicked off on Sept. 20 as congressional Democrats threatened to shut down the government in part over extending the Affordable Care Act’s enhanced premium tax credits, which help lower the cost of health-insurance premiums for roughly 22 million Americans.