Tuesday’s mess of an election in Wisconsin is the culmination of a decade of efforts by state Republicans to make voting harder, redraw legislative boundaries and dilute the power of voters in the state’s urban centers.

The Republican-dominated state legislature, which has held a majority since 2011, due in part to gerrymandered maps, refused to entertain the Democratic governor’s request to mail absentee ballots to all voters or move the primary. Then the State Supreme Court, which is controlled by conservative justices, overturned the governor’s ruling to postpone the election until June.

Now Wisconsin is conducting an election that the state’s largest newspaper — which previously endorsed Republican leaders including former Gov. Scott Walker — called “the most undemocratic in the state’s history.”

Here’s a look at how it came to this point.

Why are Republicans fighting so hard to hold an election in the midst of a pandemic?

Former Vice President Joseph R. Biden Jr. and Senator Bernie Sanders are on the ballot in Wisconsin, but the main event is the State Supreme Court race between the conservative incumbent justice, Daniel Kelly, and a liberal challenger, Jill Karofsky.

The winner will be in position to cast a deciding vote on a case before the court that seeks to purge more than 200,000 people from Wisconsin’s voter rolls — in a state where 2.6 million people voted in the last governor’s race. When the matter was first before the court in January, Mr. Kelly recused himself, citing his upcoming election. He indicated he would “rethink” his position following the April election, which comes with a 10-year term.

But the election proceeding on Tuesday is not just about the voter purge case. It is the latest example of what many in the state see as a decade-long effort by Wisconsin Republicans to dilute the voting power of the state’s Democratic and African-American voters.

Since 2011, when Mr. Walker led a Republican takeover of the state government, the G.O.P. has enacted one of the nation’s strictest laws requiring government-issued identification to vote. In 2020, a voter must have a photo ID with a current address, or an ID and acceptable proof of residency — often a hardship for poorer black Milwaukee residents who live in neighborhoods with some of the highest eviction rates in the country. A 2017 study by the University of Wisconsin found nearly 17,000 registered voters were unable to cast a ballot during the 2016 election, and untold more were deterred from voting.

The Republican majority also drew legislative and congressional boundaries that are widely considered the most gerrymandered in the country. During the 2018 election, Democratic candidates won 190,000 more votes for State Assembly seats, but the G.O.P. held a 64-35 advantage in the chamber.

Forty Republican lawmakers on Monday wrote to Gov. Tony Evers, a Democrat, asking him to reopen the state’s golf courses.

If Mr. Kelly wins, it would cement the conservative majority’s ability to block any future Democratic efforts to change voting laws and litigate an expected stalemate over congressional and state legislative boundaries during redistricting that will follow the 2020 census.

What does the Wisconsin fight mean for November?

Wisconsin is, by many projections, a key state for clinching an Electoral College victory. And in the last four years it has seen some of the closest statewide races in the country.

In 2016, President Trump won the state by less than 23,000 votes.

In 2018, Mr. Evers ousted Mr. Walker by less than 30,000 votes.

In 2019, a State Supreme Court race was decided by just 6,000 votes.

In a state so closely divided, any adjustment to voting procedures or voter eligibility has the potential to swing enough votes to tip the state.

Why did Mr. Evers wait so long to try to postpone the election?

This is truly a mystery that has consumed Democrats both inside and outside Wisconsin.

For weeks Mr. Evers insisted he didn’t have the power to change the election date without consent of the state legislature, which consistently refused to entertain the idea.

Instead he sought other remedies, such as sending absentee ballots to all voters and extending the time for voters to return ballots by mail — ideas that Republicans resisted and that were eventually struck down in courts dominated by Republican appointees.

Mr. Evers, a bland former state education secretary who won in 2018 on the backs of liberal anger and disgust with Mr. Walker, spent the run-up to the election “grandstanding with impossible demands,” said John Langeland, the Democratic Party chairman of Oneida County, in Wisconsin’s Northwoods. “He had to know that there was no way to print, distribute and tally enough ballots for an absentee-only election.”

But Mr. Evers wasn’t helped by disunity among leaders and national figures in his own party. While Mr. Sanders last week called for postponing the state’s election, Mr. Biden predicted the contest could be held safely.

Why did Democrats seek to postpone the election until June?

In Wisconsin, the pandemic is hitting hardest in Milwaukee’s black neighborhoods, which are home to a critical Democratic voting bloc. Of the 83 coronavirus deaths in the state, 33 have been black residents of Milwaukee — 40 percent of the total in a state that is 7 percent black.

Democrats had hoped postponing the contest to June and changing it to an all-mail election would alleviate fears of the pandemic and allow people to vote safely. With the state so divided, any tangible drop-off in urban turnout is likely to tip the State Supreme Court race to Mr. Kelly.

Who is actually voting in Tuesday’s election?

Public health officials across the state have closed hundreds of precincts because of the coronavirus pandemic. In Milwaukee, just five of 180 planned polling places are open, leading to hourslong lines of masked and socially distanced voters.

This comes as Milwaukee voters — an electorate that includes nearly all of the state’s black population — have lagged well behind suburban counterparts in returning absentee ballots.

During the 2018 election, the number of votes from Waukesha County made up about 57 percent of the total number of votes from adjacent Milwaukee County. Two-thirds of Milwaukee County voters backed Mr. Evers, the Democrat, while two-thirds of Waukesha County voters picked Mr. Walker, the Republican.

Absentee ballot data from the Wisconsin Election Commission shows that, as of Tuesday morning, Waukesha County voters have returned 78 percent as many absentee ballots as have Milwaukee County voters — a drastic narrowing of the voting gap between the two counties.

The data also shows that absentee turnout is far higher in the state’s major metropolitan areas than it is in the rural counties.

Statewide, 864,750 people had returned absentee ballots by Tuesday morning, according to the elections commission. But that data shows voters in urban centers were returning ballots at a far higher clip than rural Wisconsinites.

Seven counties have returned absentee ballots that account for more than 80 percent of the total turnout from the 2019 spring election, which also featured a State Supreme Court race but no presidential primary. Five of the seven are in greater Milwaukee, and the other two include Madison and La Crosse, both college towns.

Some predominately rural counties saw comparatively few absentee ballots returned. Clark County, in the state’s north, and Menominee County, whose population is largely Native American, had an absentee ballot return rate of just 17 percent of their 2019 turnout.

Bruce Johnson, the Democratic chairman in Pepin County, along the state’s western border with Minnesota, said he pushed for absentee balloting but people were less concerned because they rarely wait in lines to vote. Pepin County’s absentee ballot return rate is just 24 percent of its 2019 turnout.

“The primary risk is to poll workers and we’ve had some difficulty filling those positions because most trained workers are of vulnerable, elderly populations,” Mr. Johnson said. “Given the proclaimed inevitability of a Biden nomination and the risks involved in voting, I fear that the Republican voter suppression efforts will succeed.”