Don Beyer, a Democratic crowded running for the seat of retiring Rep. James P. Moran (D-Va.), released his third TV commercial Wednesday, and it focuses on climate change. (YouTube: Friends of Don Beyer)

Don Beyer, the best-funded candidate in the crowded campaign for the seat of retiring Rep. James P. Moran Jr. (D-Va.), released his third television commercial Wednesday, and it focuses on what Beyer calls his top priority — climate change.

“The threat of climate change is real,” he says in the 30-second ad, which will air Thursday on cable television. “That’s why I support a carbon tax, the best solution for controlling greenhouse gases.”

Beyer, who owns auto dealerships in Northern Virginia, has made climate change and a call for a carbon tax his top talking point in the campaign as he tries to separate himself from the six other candidates in the 8th Congressional District field. He, like all Democrats in this bluer-than-blue district, positions himself as a progressive, a stance that is taking some hits recently.

Patrick A. Hope, a state delegate from Arlington and founder of the Virginia Progressive Caucus, has been attacking some of Beyer’s actions on welfare reform as lieutenant governor during the past few appearances. Beyer has defended himself, but other hopefuls are beginning to question the leading candidate’s liberal bona fides, though not publicly.

The Democratic primary, which once had a dozen candidates, has shrunk to Beyer, Hope, state Sen. Adam P. Ebbin (Alexandria), Alexandria Mayor William D. Euille, former Northern Virginia Urban League leader Lavern Chatman, talk show host Mark Levine and Virginia Tech professor Derek Hyra. The 8th Congressional District covers Alexandria, Falls Church, Arlington County and part of Fairfax County.

Beyer will not be attending the Progressive Democrats of America forum tonight at National Rural Electric Cooperative Association in Arlington because of his father-in-law’s 90th birthday celebration.

The Democratic primary is June 10. The winner will face Republican Micah Edmond and Libertarian Jeffrey Carson in November.