(Reuters) - In a surprise result that showed the limits of a big-money campaign, Chicago Mayor Rahm Emanuel failed to get more than 50 percent of the vote in Tuesday's election, and must face a run-off against second-place finisher Jesus "Chuy" Garcia in April.

The setback for Emanuel, 55, came despite a visit to Chicago last week from President Barack Obama, whom Emanuel served as chief of staff.

With $6 million in his war chest at year end even after spending $4.7 million in the fourth quarter, Emanuel hugely outspent four opponents in his race for a second term as head of the nation's third-largest city.

The powerful Democrat got millions of dollars from campaign donors, including Hollywood directors and hedge fund executives, plus Obama's support.

But it wasn't enough to fend off the mild-mannered Garcia, a Cook County commissioner and progressive Democrat with a paltry campaign fund. Garcia held a low public profile before he jumped into the race last October after the teacher's union president, Karen Lewis, backed away after being diagnosed with brain cancer.

The run-off election against Garcia will be held on April 7.

With 98 percent of precincts reporting, Emanuel had 45.4 percent of the vote, while Garcia had 33.9 percent. Polls before the election had shown Emanuel close to 50 percent, and Garcia at about 20 percent.

Emanuel sounded hoarse and emotional as he told supporters to continue their fight.

"Tomorrow morning I will be seeing you at the L stops," said Emanuel "We will get back out there talking to our friends and families and neighbors as they make a critical choice about who has the strength, who has the leadership, who has the ideas to move this great city forward."

The former Illinois congressman won the mayoral seat outright when he first ran in 2011.

But controversial decisions to close 50 public schools and continued high crime helped send the Chicago mayoral election into its first run-off since the city started holding nonpartisan elections in 1995.

Garcia, 58, a former state senator and alderman, has argued that Emanuel paid more attention to the city's upper class and downtown than to the poor and communities outside the commercial center.

In a television ad, he turned one of Emanuel's apparent strengths - an ability to lure corporations to Chicago - into a negative by casting the incentive packages as tax breaks that favored big companies while taking money away from spending on police.

"We have something to say," said Garcia to his euphoric supporters Tuesday night. "We want change."

Garcia had the support of the powerful Chicago Teachers Union and its president, Lewis, who gave Garcia an immediate boost by videotaping an endorsement of him the week he announced his bid for mayor.

Suzy Broz, 26, a former Emanuel supporter, said she thought Garcia could win.

"A lot of people are upset with a one percent mayor and people feel that there's a great chance for change," Broz said.

Emanuel, known for his sometimes abrasive style, has argued that he had to make tough choices to rein in the city's budget deficit, which is expected to grow to $1.2 billion by next year due to an increase in payments to public pensions.

As the election loomed, Emanuel turned more attention to lower income residents, passing a city ordinance to raise the minimum wage.