HONOLULU — Setting up a November election showdown over a commuter rail line, pro-rail Honolulu Mayor Mufi Hannemann on Saturday was forced into a runoff with a city councilwoman opposed to the project.

Hannemann fell a hair short of winning re-election outright, taking just 49.4 percent of the vote when he needed a majority. Councilwoman Ann Kobayashi got 30 percent, and a second rail opponent, Panos Prevedouros, picked up 18 percent.

“Now it’s one on one,” Hannemann said. “We’re just delaying the inevitable. Yes, we’ll have to campaign, and we’ll do that.”

Hannemann had wanted to end the race Saturday, which would have put him in a position to move ahead with plans to build a 20-mile, $5 billion commuter rail line through the city.

Kobayashi jumped into the race as an anti-rail candidate, instead favoring alternatives like more rubber-tire buses, possibly on a new fixed guideway.

“I’ve been concerned about this rail project, how it affects our economy and people’s property taxes, and the visual impact on the city,” she said.

Without a decisive win, the anti-rail movement can claim momentum in the six weeks before the Nov. 4 general election.

Support for Kobayashi and University of Hawaii engineering professor Prevedouros indicated significant opposition to the rail plan.