UPDATE 2-US climate envoy takes aim at developing nations

* Stern says developing countries' inaction "not helpful"

* Progress "too slow" on UN climate negotiating track

* Republicans boycott Senate climate hearing again

* Bipartisan Senate group meets privately to craft deal

(Adds Senate Republican senators in paragraph 16)

By Deborah Zabarenko and Richard Cowan

WASHINGTON, Nov 4 (Reuters) - As a top American diplomat accused developing countries of inaction on global warming, a coalition of senators on Wednesday stepped up efforts to break a political deadlock that has choked U.S. steps on climate change.

Todd Stern, President Barack Obama's top climate negotiator and envoy to next month's international climate summit in Copenhagen, used blunt language in testimony to Congress when he zeroed in on developing countries' participation in talks.

Some developing countries are "hiding behind a misreading" of language in two key climate documents, the 1992 U.N. Framework Convention on Climate Change and the 2007 Bali Action Plan, which recognize different responsibilities and capabilities for rich and poor countries, Stern told the House Foreign Affairs Committee.