Some were valedictory, praising an American president’s legacy. Others lauded the courage they said it took to walk away from power. But a few world leaders struck another note over President Biden’s decision not to seek re-election: trepidation.

       “I am keeping my fingers crossed for the U.S.A. that a good president emerges from the democratic competition of two strong and equal candidates,” said the prime minister of the Czech Republic, Petr Fiala.

       With its outsize effect on governments large and small, American presidential politics is closely watched around the world, but perhaps never more so than this year. As if the prospect of a Trump redux presidency had not preoccupied foreign leaders enough, there was another urgent question in recent weeks: Would Mr. Biden even stay in the race?

       On Sunday, he gave his answer, throwing his support behind Vice President Kamala Harris, a political figure who, unlike Mr. Biden, lacks a long history of involvement in U.S. foreign diplomacy.

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       In Germany, where former President Donald J. Trump’s antipathy toward the NATO alliance and somewhat impenetrable stance on the Russian invasion of Ukraine have raised anxiety, Chancellor Olaf Scholz pointed notably toward Mr. Biden’s starkly different approach from Mr. Trump’s isolationist bent.

       “Thanks to him, trans-Atlantic cooperation is close, NATO is strong and the U.S.A. is a good and reliable partner for us,” Mr. Scholz said in a statement.

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