WASHINGTON — Seven months before the core provisions of President Obama’s health care law are to take effect, most television advertising that mentions the law continues to come from its opponents.

Since the law’s passage in March 2010, critics have spent a total of about $400 million on television ads that refer to it, according to a new analysis by the Campaign Media Analysis Group at Kantar Media, which tracks such spending. Supporters have spent less than a quarter of that — about $75 million — on ads that cast the law in a positive light, according to the analysis.

The biggest advertiser in support of the law has been the Department of Health and Human Services, which has run educational ads that mention it. Most of the negative ads have come from Republican outside groups, including Crossroads GPS, which was founded by Karl Rove and other top Republican strategists, and the National Republican Congressional Committee.

Spending on ads that mention the law dropped steeply after last fall’s presidential election — only about $4 million has been spent since then, according to Kantar, including $2.5 million on ads that are critical of the law and $1.5 million on ads promoting it. Recent negative ads have come from Republican political candidates like former Gov. Mark Sanford of South Carolina, who won a special election for a House seat last month, and from conservative advocacy groups like Americans for Prosperity.