DENVER (AP) — Democratic Sen. Mark Udall on Monday defended his support for a carbon tax, lashing back at criticism from his opponent who says plans to slash carbon emissions at coal-fired power plants raises energy costs for homeowners.

In the second debate between Udall and Republican Rep. Cory Gardner, Udall took a more aggressive stance than he has in the past, saying the changes by the Environmental Protection Agency are an opportunity for green energy companies.