There was at least one big question hanging over the French presidential elections on Sunday that could’ve affect the outcome: Would supporters of the defeated far-left candidate Jean-Luc Mélenchon vote for Marine Le Pen or Emmanuel Macron — or abstain altogether?

“For me, Sunday can have no happy outcome,” said Guillaume Delarue, a 42-year-old French expat in London, about his country’s presidential elections.

Mr. Delarue, a self-employed programmer, has been a fervent supporter of Mr. Mélenchon, whose movement, “La France Insoumise,” or “France Unsubdued,” has had harsh words for bankers, the so-called global elite and other forces it sees as the scourge of France.

Polls showed Ms. Le Pen, the far-right candidate from the National Front, far behind Mr. Macron, the independent centrist. Mr. Mélenchon had refused to endorse Ms. Le Pen but has not exactly embraced Mr. Macron.