The Senate Environment and Public Works Committee, which Boxer chairs, approved McCarthy's nomination in May, but with no Republican votes.
Asked if Boxer would support a rule change that would allow McCarthy's confirmation with a simple majority rather than the customary 60 votes, the California Democrat said, "I'd like to see her confirmed, period."
"She should get a huge majority," Boxer said. "She deserves it."
Many Republicans oppose Obama's plan to fight climate change by reducing emissions from coal-fired power plants and other sources of heat-trapping greenhouse gases. McCarthy, who's currently an assistant EPA administrator, would help implement Obama's plan.
But McCarthy has a cordial relationship with some of the industries the EPA regulates. Boxer noted that McCarthy had worked for Republicans, including former Massachusetts Gov. Mitt Romney, Obama's 2012 presidential rival.
"Who could be more suited to this job?" Boxer asked. "This woman deserves to be promoted."
Boxer wrote a letter to the nation's governors Tuesday calling for greater attention to the handling of ammonium nitrate after an explosion in April at a Texas fertilizer plant killed 14 people.
It's been more than a decade since the EPA last updated its guidelines on handling the chemical compound, and Boxer said such incidents demonstrated the need to have an EPA administrator.
"I'm very concerned," she said. "This is the longest we've had to wait."
PHOTO: U.S. Sen. Barbara Boxer listens to a speaker after taking a tour of the new terminal construction at the Sacramento International Airport on June 7, 2010. The Sacramento Bee/Hector Amezcua