62.7% of the US electricity is created using fossil fuels in 2017, half of that coal, other half natural gas. (1.3% solar, 6.3% wind)
Oil-fired and dual-fuel units in those RTOs (NYISO and ISO-NE) also make up a pretty large amount of the total installed capacity.
CO2 per energy produced per million Btu:
Natural gas 117.0
Gasoline 157.2Coal (anthracite) 228.6 Coal (bituminous) 205.7 Coal (lignite) 215.4 Coal (subbituminous) 214.3
As you can see, natural gas has half the CO2 impact per million btus as coal. It's a great bridge until wind, solar, and utility scale batteries take over (natural gas ramps quickly, coal does not, and is much cleaner).