RIVERBANK, Calif. — No state is more critical to the battle for the House in 2018 than California, and Democrats here rushed last week to tear into vulnerable Republicans over their support for the health care repeal and replace bill. Yet in a sign of how difficult it will be for Democrats to win back a House majority — even when armed with high-caliber ammunition — doubts about the lasting political potency of the Affordable Health Care Act already are emerging.Within days of the bill’s passage, focus in Washington spun to the ouster of FBI Director James Comey – and then to Trump’s disclosure of allegedly classified information to Russian officials and the appointment of a special prosecutor — while Senate amendments to the health care bill appeared likely to recast the debate long before voters turn their attention to the 2018 campaign.And the seven congressional districts targeted by the Democratic Congressional Campaign Committee hardly qualify as low-hanging fruit.