WASHINGTON — After all but endorsing the far-right presidential candidate in France last week, President Trump has been conspicuously silent since his fellow populist Marine Le Pen finished second in the voting on Sunday and put herself within striking distance of the Élysée Palace.

Mr. Trump’s reticence can be attributed to any number of factors: the White House’s reluctance to inject itself further in a foreign election; the fact that Mr. Trump is more focused on trumpeting his achievements after 100 days in office; or simply that Mr. Trump likes winners, and Ms. Le Pen looks likely to lose the runoff in two weeks.

But there is a more fundamental reason the president would do well to keep his enthusiasm for Ms. Le Pen in check. Her victory would be an acute headache for Mr. Trump, especially since he has adopted more conventional positions on NATO, the European Union and the Middle East.

“If you put ‘America First’ next to ‘France First,’ that’s a recipe for the two countries going their own way,” said Charles A. Kupchan, who was senior director for European affairs on the National Security Council during the Obama administration. “It suggests that Le Pen would not be the partner that Trump believes she would be.”