During three weeks of early voting, many of the problems Electionland has identified have been driven by higher-than-expected turnout. While experts say we won’t know if this means record-breaking turnout on Election Day, early voting in some states has already outpaced 2014, leaving election administrators struggling to keep up.

“There are two scenarios: One is that it’s been an unprecedented number of early voters, and the next is that it’s an equally historic Election Day,” said Michael McDonald, a political science professor at the University of Florida who studies voter turnout. He said that while we won’t know which is correct until Election Day, “all signs” point to higher turnout on Tuesday. “We’ve never seen this level of engagement during a midterm election,” he said.

If that happens, experts say, voters are likely to see problems at the polls that are common when turnout exceeds expectations — long lines, malfunctioning machines and new voters confused by increasingly obscure election laws.

In Austin, Texas, registration was so high that Travis County still had thousands of registration forms left to process in the weekend before early voting started. The county warned voters that those who’d registered in the final days may have to cast a provisional ballot, and its tax assessor and voter registrant, Bruce Elfant, used the occasion to advocate for online registration — something he said would have prevented the backlog. Texas is one of 13 states without online registration.

While Travis County says it has now processed all of the registrations, voters can cast a provisional ballot on Election Day if they find themselves not on the rolls. Voters should bring documentation that they are registered, if they have it, which will help poll workers process their ballot.

In Johnson County, Kansas, the surge in voter registration ahead of the Oct. 16 deadline was so large, the county’s election infrastructure couldn’t keep up, leaving some who had already registered confused as to why they weren’t showing up on the county’s voter registration database. The county hired more than 40 employees to process the upswing in applications, according to Ronnie Metsker, the county’s election commissioner.

“We had an enormous surge of registration activity, and the department was overwhelmed by mail application traffic,” Metsker said. “When we saw our team was unable to keep up with the pace of it, we brought in more staff. Then we kept increasing it.”

To prepare for high turnout, Johnson County set up the maximum number of machines, and election administrators have been crafting fallback plans should certain polling locations have particularly long lines. Still, there are limits to what the office can do in the face of high turnouts. “We only have so much hardware,” Metsker said. “It comes down to human capacity and infrastructure capacity.”

Tommy Doyle, the supervisor of elections for Lee County, Florida, said the turnout resembles that of a typical presidential year. Lee County has already received close to 140,000 mail-in ballots — inching closer to the 156,000 received in 2016.

“This is similar to a presidential election,” Doyle said. “I think our numbers are going to be really close to 70 percent turnout.” Lee County hired 35 temporary workers to process mail ballots and prepare for Election Day voting, and it implemented a new system to automate some of the work.

Turnout for earling voting at some polling locations in Ada County, Idaho, was so high that county election officials began encouraging voters to go to other, less crowded sites.

If turnout is unexpectedly high on Tuesday, voters may face longer than usual lines. Experts say that wait times longer than a half hour will cause some voters to abandon the effort. However, most states allow every person already in line when the polls close to vote.

With so many confusing election laws, ProPublica created a state-by-state explainer for voters on Election Day, breaking down what you need to know on voter registration, voter ID laws and access to the ballot.

ProPublica’s Electionland project covers problems at the polls that prevent people from voting. If you experience or witness something on Election Day, let us know at https://www.propublica.org/electionland.