Foreign officials who have dealt with Mr. Rubio since he became Mr. Trump’s top diplomat have described him as downplaying some of Mr. Trump’s wilder ideas and translating them into more realistic policy approaches, although they also question whether he truly speaks for a president with whom he does not have a close personal relationship.

And there is only so much Mr. Rubio can do to sugarcoat Mr. Trump’s agenda, which is driven by a view that Europe economically exploits the United States, is culturally out of sync with the values of Mr. Trump’s political movement and must do business with Russia’s president, Vladimir V. Putin.

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