PARIS — 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.

Mr. Mélenchon’s critics say his obstinacy is petulant, wounded pride that can only help Ms. Le Pen’s National Front. But it also speaks to the passions that Mr. Macron, a seemingly mild-mannered centrist, provokes in large parts of the French electorate, far left and far right, who share a view of the 39-year-old former investment banker as a fire-breathing incarnation of evil market culture.

As populism and anger over the impacts of globalization energize much of the electorate, Mr. Mélenchon’s stand has added a new element of uncertainty into the final round of voting on May 7.