During the Democratic primary, environmental groups spent nearly $1.8 million in outside money to help Markey defeat Stephen Lynch.
And without a so-called People’s Pledge in effect during the sprint to the June 25 election, that spending could translate into television, radio and internet ads.
Markey and Lynch agreed to the pledge, which helped discourage outside campaign ads in the primary.
At issue for many environmental activists is the fate of the Keystone XL oil pipeline, which Markey opposes but Gomez supports.
Others point to what they say is Markey’s long record championing environmental causes in Congress and combating climate change.