Chicago is about to elect its first black female mayor.

In a runoff election on Tuesday, the city will elevate either the former police board president Lori Lightfoot or the Cook county board president, Toni Preckwinkle, to the highest office in the city.

Lightfoot, who would also be the city’s first openly lesbian mayor, is a heavy favorite to replace the outgoing mayor, Rahm Emanuel, in recent polls. Her message of political reform seems to be resonating with Chicago voters, particularly in the wake of the arrest of a powerful longtime alderman connected to Preckwinkle.

“That refocused the election,” said Dick Simpson, a professor of political science at University of Illinois at Chicago and a former alderman who endorsed Lightfoot.

Preckwinkle – along with the Illinois comptroller, Susana Mendoza, and Bill Daley, Barack Obama’s former chief of staff whose father and brother ruled over the city for a combined 43 years – had led the crowded pack of mayoral hopefuls, in part on the basis of her significant experience in Chicago politics.

But the arrest in early January of Ed Burke – the longest-serving Chicago alderman and a powerful cog in this city’s political machine – on extortion charges threw a wrench into the mayoral race. Burke was connected with Mendoza, Daley, Gery Chico and Preckwinkle, who were the four candidates with the deepest pockets.

Preckwinkle faced particular scrutiny, as one of the charges against Burke related to a $10,000 contribution to her campaign, which the longtime alderman had allegedly muscled out of a fast-food restaurant owner in his ward. Preckwinkle disavowed Burke and continued to lead in polls in the run-up to the February election, but Lightfoot – who has never held elected office – won the most votes in the first round, 17.5% to Preckwinkle’s 16%.

“[Burke’s] corruption made sure that the focus of the election went toward change and reform,” Simpson told the Guardian in a phone interview on Monday. “And while Preckwinkle has some reform credentials ... she isn’t proposing the same kind of reform platform as Lightfoot.”

No matter who wins Tuesday’s runoff, history will be made.

Chicago has elected just one black mayor in its history: the late Harold Washington, who led the city from 1983 until his death in office in 1987. The city has also had just one female mayor: Jane Byrne, Washington’s immediate predecessor from 1979 to 1983. But the city has never elected a black woman nor an openly LGBT individual to serve as its mayor.

The race has, at times, gotten testy, with the two candidates hitting one another with tough attack ads and sometimes personal insults. Lightfoot has accused Preckwinkle of “blowing some kind of dog whistle” to conservative Chicagoans about her sexual orientation, and in one debate called Preckwinkle “pathetic” and accused her of lying. Preckwinkle and her allies, meanwhile, have attacked Lightfoot over her role on the police board, on which she served until 2018.

Emanuel appointed Lightfoot to the police board and to the new police accountability taskforce in 2015. The board under her leadership increased disciplinary action against officers. Lightfoot’s taskforce delivered withering criticism of Emanuel’s handling of the murder of Laquan McDonald by Jason Van Dyke, a white former police officer – the scandal that many here believe torpedoed the mayor’s chances of being re-elected.

But, in this city, where many residents – particularly black and brown Chicagoans – have a fraught relationship with police, Lightfoot’s history in law enforcement, and her ties to Emanuel, may raise suspicions.

Chance the Rapper, whose father chairs Preckwinkle’s campaign, is among prominent figures who have warned that Lightfoot’s record “has not been in the best interests of young black people in Chicago”.

The US representative Bobby Rush, who backs Preckwinkle, went even further at a rally recently, suggesting to a crowd in the city’s predominantly black southside that “the blood of the next young black man or black woman who is killed by the police is on your hands” if you vote for Lightfoot.

Lightfoot, who leads by more than 30 points in a recent poll, has shot back, calling her opponent “desperate”.

The rhetoric has grown so heated that the civil rights leader Jesse Jackson called on the candidates to hold a post-election unity event; both have agreed to do so.

Regardless of its outcome, Tuesday’s election will probably be seen as a step away from the politics of Chicago’s past, as both candidates have positioned themselves as progressive agents of change.

“Tuesday, the race will be over,” Jackson told an audience in the city. “The healing must begin.”