The bill, set for a Senate vote this coming week, would spend about $2.8 billion per year on conservation, outdoor recreation and park maintenance.
McConnell told the two senators, who are both seeking reelection this year, that he would not consider the bill unless Trump was on board.
Cantwell credited Sen. Richard Burr, R-N.C., for forcing attention on the conservation fund by blocking a 2018 spending bill that did not renew the program.
While widely supported, the outdoors bill faces sharp opposition, mainly from Sen. Bill Cassidy, R-La., and other Gulf Coast senators pushing to ensure it includes revenue-sharing for their states from offshore drilling.
Calling Trump's record on the environment "the worst one in history,'' Cantwell said Gardner, Daines and other Republicans will be forced to defend the president as they go before voters.