UPDATE (Nov. 7, 2016, 7 a.m.): The SEPTA strike was settled early Monday morning.

While commentators digest the latest announcement from FBI Director James Comey, a story with the potential to have more of an impact on the election is playing out with little notice in Philadelphia. Last Tuesday workers for the city division of the regional transportation authority, SEPTA, began a strike over a new contract. The strike has shut down the city’s buses, subways and trolleys, and snarled the city’s roads since then.

Last Friday, a Philadelphia judge declined to issue an injunction ending or suspending the strike, but she scheduled a hearing for 9:30 a.m. Monday to take up the strike’s potential impact on the election. The evidence on the effects of prior transit strikes is limited, but given what we know about Election Day in Philadelphia, the people who rely on the city’s public transit network, and about voting in general, the potential impact on residents’ ability to vote could be substantial. And that impact is likely to be concentrated on residents of color, as well as on Philadelphia’s poorer residents.

The nation’s fifth-largest city, Philadelphia is the largest city in any swing state. There is also no city as populous as Philadelphia with a larger share of residents in poverty. It is not surprising, then, that Philadelphia relies heavily on its public transit network. As it is elsewhere, that reliance is particularly heavy in poorer communities and communities of color. Below, for instance, data from the 2014 American Community Survey shows the relationship between the share of census tract residents who are black and who ride public transit to work in Philadelphia. The relationship is substantial: If we go from a census tract with no black residents to one that is entirely black, we should expect the share of people using public transit to get to work to rise by 27 percentage points.

Or consider how the percent riding public transit correlates with a census tract’s median household income (the panel on the right). Here, the correlation is strongly negative: As census tracts become wealthier, they become less dependent on public transit. Imagine moving from Philadelphia’s first-quartile census tract (with a median household income of $25,600) to its third-quartile Census tract (where median household income is $52,270) — public transit ridership should drop by 9.6 percentage points. This relationship is likely to make sense to people familiar with the city’s demographics, as some of the wealthiest neighborhoods are in and around the city’s commercial center. The effects of any Election Day disruption to transportation are likely to be felt disproportionately in the city’s outlying neighborhoods.

The impacts of the strike are predictable: Without the buses, subways and trolleys — yes, there are really trolleys — people commuting into Center City get up earlier to drive, bike or walk to work. But that strategy also has the potential to mean that many voters on Tuesday will face an unenviable choice: Vote when the polls open at 7 a.m. or get a jump on the trip downtown. They’ll also know that lots of other people are facing the same choice, a fact likely to produce lines at many polling places. Will that, in turn, dampen voter turnout?

That’s certainly the fear of city officials. On Sunday night, the city filed suit to suspend the strike and voiced the concern that an “Election Day strike will make it practically impossible for many Philadelphians to participate in this election.”

Extensive research on voter turnout suggests that the city is right, and that voters are more likely to vote when it is more convenient to do so. Voting is to some extent a habitual behavior, so people are less likely to vote when their habits are disrupted. When Los Angeles County consolidated its polling places for the 2003 gubernatorial recall election, for example, in-person voting dropped by a sizable 3.03 percentage points in precincts that were relocated compared to those that were not. That decline was partially offset by increased absentee voting, but Pennsylvania has no early voting, and the deadline for absentee ballot applications has come and gone.

Philadelphia has actually had a strike during an election before, in 2009. At the time, voters were choosing a district attorney and controller, as well as several judicial posts. In 2009, some 122,946 voters cast ballots for district attorney, a number that was actually up from the 120,424 voters who cast ballots for district attorney in 2005. But both were paltry turnouts for low-profile elections, and turnout dynamics in more prominent elections can be very different, as Temple University professors Kevin Arceneaux and David Nickerson have demonstrated. For every one Philadelphia voter in 2009, there were 5.6 in the 2012 presidential cycle, and absent a strike, we might expect a similar number this Tuesday. The 2009 election is accordingly a poor guide to the would-be impacts of the current strike.

When voting gets easier, turnout increases disproportionately among people who don’t always vote, as evidence from all-mail elections demonstrates. On the flip side, when voting gets harder, those who aren’t habitual voters are more likely to stay home. Poorer voters are less habitual voters. So a disruption as significant as an ongoing public transit strike poses a real threat to turnout on Tuesday.