Sen. Jeff Flake, R-Ariz., walked out on the top Democrat of the Senate's environment subcommittee after saying he opposed overly strict Environmental Protection Agency rules at a hearing Wednesday.

Sen. Tom Carper, D-Del., the ranking member on the clean air panel of the Environment and Public Works Committee, asked Flake to wait for him to finish a quick procedural matter before speaking.

Carper motioned to enter statements from environmental groups into the hearing's record, but when he looked up Flake had disappeared.

"Oh! It looks like Sen. Flake has left," Carper said, jokingly saying it would be the last time he entered anything in the record. "I am not unsympathetic to the issues he raised," the Democrat from Delaware noted.

It is unclear why exactly Flake left. Flake's office did not immediately respond to an email asking why he had left so abruptly after Carper had asked him to stay. Flake had just shown up to present his comments on EPA regulations, and hadn't been in the hearing room when the panel commenced.

But it could have been what Carper was submitting in the record. The environmentalists' statements that Carper entered into the hearing's record expressed opposition to legislation the senator from Arizona had told the panel only moments before he is supporting to hold back EPA's implementation of the rules.

The hearing of the clean air subcommittee Wednesday was held to begin the review process for legislation looking to hold back implementation of the Environmental Protection Agency's 2015 ozone standards for smog.

Representatives from Flake's state said at the hearing that the overly strict regulations would be nearly impossible to meet while driving out investment dollars from the state.

Carper defended EPA, saying it gets sued if it doesn't reevaluate and tighten its regulations.