Sarah Kliff thinks she knows why President Obama spent so little time defending the Affordable Care Act during his State of the Union address Tuesday night: doing so “gives weight to the threat of repeal, recognizes it as legitimate,” she writes. “Obama actually has some company: President Lyndon B. Johnson didn’t even mention Medicare in his 1966 State of the Union address, which happened just 12 days after the new entitlement programs for seniors rolled out. In his 1967 speech, he mentioned the program just twice.” But health care reform is probably a bigger target for the GOP presidential candidates and House Republicans — who have pledged to go after the law piecemeal — than Medicare ever was. Obama will have to defend and re-sell the measure on the campaign trail, point to the seniors, young adults, and sicker Americans who are already benefiting from its provisions and separate the actual legislation from the political process out of which it was born. If he succeeds in delivering that message, then we’ll be able to compare reform to Johnson’s achievement in signing a law that has become the very bedrock of the American safety net.