WASHINGTON — Though many of its rules will not take effect for months, President Obama’s health care law is already the subject of an aggressive advertising campaign by Republicans to sow doubts about how it will work.

In one of the largest campaigns of its kind, Americans for Prosperity, a conservative advocacy group financed in part by Charles and David Koch, will begin running television commercials this week asserting that the law will limit Americans’ health care choices.

The group is spending more than $1 million on the campaign, which will initially include television advertising in Ohio and Virginia, along with online ads asking people to test their “Obamacare risk factors.”

Republicans have staked much of their near-term political success on the bet that the health care overhaul will be unpopular with Americans as it is implemented in a process that they have warned will be chaotic and frustrating. Many Republicans in Congress have said they would push to repeal the law.