As Mitt Romney moves closer to locking up the Republican presidential nomination, the questions about his ability to attack President Obama on healthcare are only getting louder.

Conservatives have long been uneasy with Romney’s record on healthcare. But as an Obama-Romney matchup becomes increasingly likely, so does the prospect that Republicans won’t be able to aggressively campaign against a law that helped propel their historic wins in 2010 — and hasn’t gotten much more popular since.