Senate Majority Leader John Thune can afford to lose three Republican senators and still pass the bill, and there are more than that, right now, who have problems with it. Like the House, he will have to balance the concerns from moderate and conservative members of his conference.

Senate Majority Leader John Thune can afford to lose three Republican senators and still pass the bill, and there are more than that, right now, who have problems with it.

Republicans said California’s rules were overbroad and would effectively set a national standard because California is the largest car market in the United States and under the Clean Air Act, other s...