To the list of worries about the euro zone, add one in bold: the fate of France as it heads into the first round of a closely contested presidential election this weekend. After a long stretch in which President Nicolas Sarkozy grappled with the euro crisis, investors are now wrestling with the implications of a potential victory by Socialist candidate, François Hollande. Mr. Hollande’s pledges of higher taxes on the rich, and higher government spending, are luring voters disenchanted by the austerity medicine Mr. Sarkozy administered in hopes of protecting France from financial contagion. Many investors, though, are questioning what...