The environmental group, which has endorsed Markey, is spending $400,000 to send mailers to 150,000 Massachusetts households touting Markey's work on expanding clean-energy jobs and reducing U.S. dependence on foreign oil.

In contrast, the mailer charges that Gomez "is in the pocket of Big Oil."

The League of Conservation Voters was heavily involved in the primary, spending nearly $850,000 on a field campaign to support Markey over Rep. Stephen Lynch (D-Mass.).

Markey leads Gomez in every poll of the race, but Gomez had kept the race to single-digits for much of the general. The two candidates will meet for their first televised debate on Wednesday night, just under three weeks from Election Day.