After heading into elections with high hopes, Turkey’s political opposition is struggling to fight off despair and plot a course to give their candidate a fighting chance against the incumbent, Recep Tayyip Erdogan, in a runoff later this month.

While Mr. Erdogan, bidding for a third five-year presidential term, failed to win a simple majority in Sunday’s election, he still led the opposition by a margin of about five percentage points. That, and a number of other indications, point to a win for the president in the second round on May 28.

Importantly, Mr. Erdogan looks likely to be the primary beneficiary of votes from supporters of an ultranationalist third candidate, Sinan Ogan, who has been eliminated despite a surprisingly strong showing over the weekend. The first-round results pointed to growing nationalist sentiment across the electorate that will probably boost the president.

All of that amounts to an uphill battle for the challenger, Kemal Kilicdaroglu, who heads a six-party coalition that came together with the goals of unseating Mr. Erdogan, restoring Turkish democracy, righting the economy and smoothing over frazzled relations with the West.