The German chancellor, Angela Merkel, has cast the agenda of the Group of 20 summit meeting as a stark contrast to Mr. Trump’s America First approach, particularly on climate change. She has called the Paris accord “irreversible,” and diplomats have expressed hope that the 19 other countries would make it clear that their support is unwavering. Environmental activists, hoping to highlight America’s status as an outlier, also are pushing hard for a united front against Mr. Trump.

In recent days, however, those aiming to isolate the United States on climate issues have softened their language to say they hope an “overwhelming majority” embrace the Paris agreement. Saudi Arabia has indicated it is unlikely to climb on board and Russia, Turkey and Indonesia are sending mixed signals about how forcefully they will declare their support for the Paris deal.

“Huge efforts are underway now to make sure as many countries as possible hold the line and compensate for America’s withdrawal by redoubling their efforts. How far this goes, I have my doubts,” said Dennis Snower, president of the Kiel Institute for the World Economy, a leading German think tank advising the European Commission ahead of the summit meeting.

“It doesn’t look good,” Mr. Snower said. “It does not look like we are going to have 19 countries and the United States against.”

The Group of 20 meeting is the first high-level diplomatic gathering since Mr. Trump announced last month that America would exit the Paris agreement. How full-throated a case other rich nations are willing to make for the climate deal now could set the tone for years to come.