Environmental advocates want Missouri Sen. Claire McCaskill along for the ride as they try to get climate-change legislation through Congress.

Representatives from the Washington-based organization Environmental Defense came to St. Joseph with their pitch Tuesday, flattering the freshman lawmaker as a “majority maker” and urging Ms. McCaskill’s support for a law putting a cap on greenhouse gas emissions.

“McCaskill’s important because if she signals her support for this, she could bring others along with her,” said Bill Petty, a national field director for the organization. “She is a bellwether in many ways.”

The legislation at issue is called the Lieberman-Warner Climate Security Act, a measure that aims to reduce U.S. greenhouse gas emissions with an eye for safeguarding growth in the American economy.

A centerpiece of the act is its cap-and-trade mechanism, a system that abides by an overall cap but allows businesses with excess gas emissions to buy “carbon credits” from companies that operate below their output limits.

Not everyone agrees with the approach. The Wall Street Journal referred to one incarnation of the system as “cap-and-charade.” Dr. Margo Thorning, an executive for the American Council for Capital Formation, said the Lieberman-Warner legislation could reduce the gross national product by nearly 3 percent when fully in place.

Ms. McCaskill, a Democrat, holds no dire views of the legislation but has yet to arrive at a comfort level.

“I am supportive of a cap-and-trade system to help address the world’s climate change problems, but I still have serious concerns about how it might affect lower-income Americans,” she said in a statement released Tuesday. “Any policies we put in place must limit the financial impact on low-income households.”

Mr. Petty said the Lieberman-Warner bill, now out of committee and headed for Senate debate after the Memorial Day recess, takes a market-based approach by spreading its provisions over a range of carbon producers. Rather than stifling economic output, he said, it holds the potential to create jobs that come with “green technologies.”

“America is either going to be buying those products or they’re going to be developing and selling them to the rest of the world,” Mr. Petty said.

Not only large industries stand to be impacted. Farm operations that work to reduce emissions, as with methane capture and reforestation, can trade credits to businesses not meeting their limits.

“This is purely additional revenue for agriculture,” said Tony Kreindler, Environmental Defense media director. “You can pay farmers for reductions you’re having trouble making at the plant.”

Ken Newton can be reached

at kenn@npgco.com.