DENAIN, France—Emmanuel Macron and Marine Le Pen traded blows at a distance on Monday over who would best protect French voters’ purchasing power, kicking off a tense fight to win the presidential election runoff on April 24. Macron, a pro-European economic liberal who became president in 2017 after easily beating Le Pen, is facing a much tougher challenge this time. He is slightly ahead in polls, but Le Pen, who ahead of Sunday’s first round successfully tapped into anger over the cost of living and a perception that Macron is disconnected from everyday hardships, pressed on those points on Monday. “Emmanuel Macron, if by some mischance he was reelected, would feel totally free to continue his policy of social wreckage,” Le Pen said on a visit to a rural area southeast of Paris. Warning of the “dark clouds” that inflation cast on France, she said Macron had failed to protect …