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WASHINGTON — When Andrew Wheeler, President Trump’s nominee to lead the Environmental Protection Agency, testified before Senators on Wednesday at his confirmation hearing, he found himself walking a tightrope on the issue of climate change.

One of the most pointed moments came shortly after he told senators that climate change was not “the greatest crisis” facing our planet. Senator Jeff Merkley, Democrat of Oregon, then asked Mr. Wheeler, a former coal-industry lobbyist, to rate his level of concern on a scale of one to 10.

After a pause, Mr. Wheeler said, “about eight or nine.”

“Really?” Mr. Merkley responded.

Senator Merkley and other Democrats on the Senate Committee on Environment and Public Works took pains to focus in on Mr. Wheeler’s efforts to roll back environmental protections and undo Obama-era regulations designed to fight climate change as indicators that he is unsuitable to lead the E.P.A.

President Trump formally nominated Mr. Wheeler last week to lead the agency but he has been on the job in an acting capacity since his predecessor, Scott Pruitt, resigned in July amid ethics scandals. If Mr. Wheeler is confirmed, a strong likelihood with Republicans holding a 53-to-47 majority in the Senate, he will formally become one of the top soldiers in Mr. Trump’s battle to undo regulations.