The April sun beats down as hundreds of supporters wait at a west side church to hear Jesús "Chuy" García, the county commissioner and former legislator who is looking to unseat Chicago mayor Rahm Emanuel. In an effort to drum up support among African American voters, García is flanked by black leaders like Reverend Jesse Jackson Sr, the philosopher Cornel West and Karen Lewis, president of the Chicago Teachers Union, which has given the Mexican American candidate more than $300,000 in campaign donations to date.

In February, Chicagoans pushed a sitting mayor into a runoff – a first in the city's history. The vote will take place on Tuesday, and it may now hinge on support from African Americans, who represent a large percentage of the city's undecided voters. Four years ago, after leaving his position as President Barack Obama's chief of staff, Emanuel rode into city hall on such support, picking up 58% of the African American vote and capturing every black ward in the city.

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But on re-election night, Emanuel took just 42% of the black vote – a phenomenon many attribute to factors including the mayor's unprecedented closing of almost 50 schools, his consistent allocation of city funds to resource-rich downtown neighborhoods, and what some say is an abrasive, closed-off personality.

A recent poll of Chicago's African American voters showed Emanuel with a 10-point lead over García, though 18% of those voters remained undecided. That is why for the past month both candidates have courted black leaders, run TV ads featuring black police officers, and ultimately painted themselves as the candidate who cares about poverty and unemployment – issues affecting voters on the city's predominantly black south and west sides.

García told the Guardian that poll didn't faze him.

"Not at all", he said on Friday. "On election day, we'll have 6,000 campaign volunteers knocking on doors. And remember, most of the polling done has always had us behind. That's why we're in a runoff and that's why I expect to win on Tuesday."

Both Emanuel and García have had their share of success and failure.

In March, an Instagram photo of Emanuel posing next to a straight-faced African American man exploded on social media, after it was picked up by media sites such as MSNBC and Bloomberg.

The photo's caption, written by health store worker Albert Griffith, read: "The Mayor of Chicago came by my job, still can't stand this muthafucka tho."

Griffith said he could not stomach many of the mayor's policies, especially the decision two years ago to close almost 50 schools.

"Most of those schools that closed was either on the south side or the west side, none on the north," Griffith told the Guardian. "Now you wanna be all happy-go-lucky with African Americans, but where was you before the election? A lot of violence has happened on the south side. Where was you at?"

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While Griffith vehemently opposes Emanuel, he said would not be voting for García either – apathy that suggests anger toward the mayor doesn't equate to a win for his opponent.

García has netted key support from African American leaders. In late March, he held a luncheon for 50 prominent African American women. The next week, Emanuel outdid him, inviting more than 1,500 African-American women to a free lunch hosted by notables like former Obama social secretary Desirée Rogers and Rochelle Trotter, wife of late Chicago culinary legend Charlie Trotter.

Janette Wilson, of Jackson's Rainbow PUSH Coalition, who attended García's lunch, said his ability to mobilize voters on the ground was key to his strategy in February and will be crucial again in turning out black voters.

"What Rahm Emanuel owns is the air – he's purchased the air," Wilson said. "But this election will be won and fought on the ground, and I think that is the question. Who can get the people out to the polls? If there's a tremendous turnout in the African American community, that will go in favor of Jesús García."

Prior to February's election, Emanuel raised more than $16m to García's $1.3m. Since election night, as of 29 March, the two have each spent more than $1m on TV and radio ads, according to the political data site Aldertrack.

But as much as Emanuel has drummed up divisions in Chicago's African American neighborhoods, he has had no trouble courting the city's black leaders. In February, he picked up support from the controversial Father Michael Pfleger of St Sabina's, a heavily African American Catholic parish on the city's south side, and was endorsed by US congressman Bobby Rush, an unexpected move considering Rush's past criticism of Emanuel's school closings.

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The Reverend Jesse Jackson Sr leads voters to the polls in the run-off for mayor of Chicago, in support of Jesús Chuy García. Photograph: Chloe Riley

So far, there have been more than 100,000 early voters – almost double the number who voted prior to February's election. A recent citywide poll from Ogden and Fry had the mayor with a 13-point lead over García, with 18% of voters still undecided.