We find that pending federal emission standards more than low natural gas prices are promoting a shift from coal to natural gas as the dominant fuel for U.S. power generation. Similar to a number of recent studies, we show that low natural gas prices have made 9% of current coal capacity more expensive to run than a median-cost natural gas plant. However, we find that another 56% of coal capacity would become just as costly if the pending emission standards are enacted, a much higher fraction of economic vulnerability than has previously been reported. We also present a novel set of cost plots which show that coal plants would again become the dominant least-cost generation option should the ratio of natural gas to coal prices rise slightly to 1.8 from its current level of ~1.5. If the stricter emission standards are enforced, however, natural gas plants will remain cost competitive with a majority of coal plants even if natural gas becomes more than four times as expensive as coal.