The far-right leader Marine Le Pen faces an uphill battle in France’s presidential runoff, less than two weeks away. But she saw daylight through a small window on Tuesday, and from an unlikely source: her defeated counterpart on the far left.

Alone among all of France’s major political personalities, Jean-Luc Mélenchon, the leader of his own “France Unsubjugated” movement, who finished a strong fourth in Sunday’s voting, has refused to endorse Ms. Le Pen’s opponent, the former economy minister Emmanuel Macron.