On Election night, Chicagoans watched as Cook County Commissioner Brandon Johnson narrowly defeated former Chicago Public Schools CEO Paul Vallas to become the next mayor of Chicago.

Johnson won with 52% of the vote to Vallas’ 48%, pulling ahead with a margin of roughly 26,000 votes. Even with an estimated several thousand more mail-in ballots en route, the Associated Press called the race for Johnson around 9:30 p.m. on election night.

A WBEZ analysis of this week’s election results breaks down how Johnson won, how Chicagoans voted and what voter turnout looked like among different groups. For the analysis, WBEZ used 2020 population figures from the U.S. Census Bureau to determine the racial and ethnic makeup of Chicago’s nearly 1,300 precincts. WBEZ also compared election results and voter turnout from the runoff election with similar data from the February general election.

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The Chicago Board of Election Commissioners will continue to count ballots they receive that are postmarked by April 4 until April 18.

Johnson gained ground while Vallas hit a “ceiling”

Heading into the runoff, a looming question for both candidates was who would sway the voters who chose a different candidate in the February general election. In the runoff election, most of these votes that were considered “up for grabs” went to Johnson, proving decisive in his slim victory over Vallas.

In the February general election, Johnson and Vallas were the top two candidates in a field of nine mayoral candidates, the two receiving nearly 310,000 votes combined. In the runoff, there were about 610,000 votes cast for mayor. Assuming voters stuck to who they voted for in February, Johnson and Vallas gained about 300,000 additional votes in the runoff, of which two-thirds went to Johnson, according to a WBEZ analysis of precinct-level election data. This was largely driven by Johnson’s sizeable wins in precincts that incumbent Lori Lightfoot led in February.

In the runoff, Johnson won every precinct that Lightfoot led in the general election, capturing about 112,000, or nearly 80%, of the votes in those precincts. Johnson also won 80% of the votes in precincts Willie Wilson led, although they represent a much smaller share of the vote. The two candidates split the votes in Congressman Jesús “Chuy” García-led precincts from February, with slightly more than half going to Vallas.

Overall, Johnson won more than 730 precincts in the runoff, about 500 more precincts than he led in February. Vallas, however, may have hit a “ceiling” in the runoff, picking up about 65 more precincts in the runoff election compared to the February general election.

Support in Black precincts underpinned Johnson’s win

Between the general and runoff elections, both Johnson and Vallas gained a similar number of votes in majority-Latino and majority-white precincts. Johnson gained nearly twice as many votes than Vallas did in precincts with no racial majority. But it was Johnson’s wins in majority-Black precincts, where he gained about 88,000 more votes than he had in February, that allowed him to erase Vallas’ 60,000-plus vote lead from the general election.

Johnson almost swept majority-Black precincts, where he won 80% of the votes, while Vallas swept majority-Asian precincts, getting nearly 80% of the votes there. Vallas captured more of the votes in majority-white and majority-Latino precincts, but the margins were closer between the two candidates.

Vallas won 60% of the votes, to Johnson’s 40%, in majority-white precincts. Vallas won nearly all the majority-white precincts on the far Northwest and Southwest sides of the city, and most of the majority-white precincts downtown and extending north through the Near North Side, Lincoln Park, Lakeview and North Center communities; and extending south and west a bit into the Near South Side, Near West Side and West Town communities.

Johnson support from majority-white precincts came largely from precincts on the near Northwest Side and along the lakefront on the far North Side, including parts of Edgewater, Lakeview, Logan Square, Rogers Park, Uptown and West Town.

The vote in majority-Latino precincts was closer between Johnson and Vallas, with Vallas winning 54% of the votes to Johnson’s 46%. Interestingly, on the Southwest Side, the vote in majority-Latino precincts was split by the Stevenson Expressway. Vallas won most of the precincts south of the highway — in the Archer Heights, Brighton Park, Gage Park, West Elsdon and West Lawn communities — with his support growing in precincts further to the west.

Meanwhile, Johnson dominated in the majority-Latino precincts north of the Stevenson in the Pilsen and Little Village communities, where the endorsement of García might have bolstered his appeal.

Changes in voter turnout benefitted Johnson

Preliminary citywide turnout stands at 38.5% and is expected to increase over the next few days as more mail-in ballots get counted. Preliminary turnout has already surpassed turnout in the February general election, which was 36%.

Collectively, turnout in precincts that Johnson won was lower than it was in precincts that Vallas carried — 36% compared to 43%. Despite the lower turnout, Johnson benefitted from runoff turnout staying roughly consistent with general election turnout in places where the most votes were up for grabs — precincts led by Lightfoot in the February election. In the 2015 and 2019 mayoral runoff elections, turnout was typically lower in precincts that were carried by mayoral candidates in the February elections of those years who did not make the runoff.

Overall turnout increased slightly between the February general and April runoff elections in precincts that Lightfoot carried in February, precincts that went heavily to Johnson in the runoff.

Younger voters made their mark

Overall, younger voters are still underrepresented in the electorate relative to their share of the population. But in the runoff, at least 6,000 more Gen Z and younger millennial voters cast ballots compared to the general election.

The increase is based on preliminary turnout reported at the end of April 4 and it’s unclear how ballots cast by voters 35 years old and older will change. If patterns from February hold, there may be a higher percent change in ballots cast for voters 25 to 34 years old and for those 35 to 44 years old than other age groups. Among mail-in ballots that came in after the Feb. 28 general election, at least twice as many ballots were received from voters ages 25 to 34 and 35 to 44 compared to other age groups.

Where are the remaining ballots?

In February’s election, roughly half of the outstanding mail-in ballots as of Feb. 27, the day before Election Day, were returned and counted, according to Max Bever, director of public information at the Chicago Board of Election Commissioners. Assuming a similar rate of return, there will likely be about 45,000 ballots returned of the 90,000 outstanding mail-in ballots as of April 3, the day before the runoff election.

As of April 12, about 54,000 of those outstanding mail-in ballots have been received by the elections board, according to Bever. There are now 34,000 outstanding mail-in ballots that have not been returned.

The 32nd, 44th, 47th, 2nd, and 1st wards — all on the city’s North Side or downtown — are the top five wards with the highest number of unreturned mail-in ballots. The 32nd and 2nd wards went for Vallas and the 44th, 47th and 1st wards went for Johnson.

Amy Qin is a data reporter for WBEZ. Follow her at @amyqin12. Alden Loury is the data projects editor for WBEZ. Follow him at @AldenLoury.