A Bernie Sanders campaign rally in Chicago. Scott Olson/Getty Images One week ago, Sen. Bernie Sanders pulled off a seemingly impossible victory over Democratic presidential frontrunner Hillary Clinton.

He's now potentially in position to pull off more than one such upset on Tuesday.

Polling in Ohio showed that Clinton held as much as a 30-point lead just before last week's Michigan primary, while she was shown to have a lead of more than 40 points in Illinois in another poll.

But since the shocking result in Michigan, where Clinton narrowly lost to Sanders despite entering with a 20-point lead in the polls, Clinton's edge in Ohio has narrowed to within 10 points, while Sanders has even managed to pull ahead of Clinton in one Illinois poll.

Both states vote Tuesday, in addition to Florida, North Carolina, and Missouri. Clinton has maintained a large advantage in Florida and North Carolina, while Sanders was shown to have a slight edge in Missouri.

Michael Burton, a political science professor at Ohio University and a former assistant to the chief of staff for Vice President Al Gore, told Business Insider that the Michigan and Ohio electorates were fairly similar. (And the same can be said of Illinois.)

"Michigan was kind of like the 1987 stock market crash," Burton said. "For all the distinctions that people inside Ohio like to make from Michigan, they don't look that different."

Clinton's prior support of trade deals such as the North American Free Trade Agreement and the Trans-Pacific Partnership may continue to hurt her along the Rust Belt, as Sanders has campaigned increasingly hard on trade issues. In Michigan, where many trade-based and manufacturing jobs were lost since the passing of Nafta in the 1990s, trade issues clearly played a role in the race.

"If you look at where some of the massive failings [of polling] have taken place, they've happened in swing states," Burton continued, adding that many Ohio voters he had spoken with were "rejuvenated" on Sanders after his Michigan upset.

The polling within Ohio and Illinois found that "the same thing might be happening" in other parts of the Midwest that happened in Michigan, John Green, the department chair for the political science department at the University of Akron, told Business Insider.

"It's a much more competitive race than we would've thought of 10 days ago," Green said.

He added that, after the Michigan contest, students who had been saying they would vote for Clinton because they wanted to "go with the winner" said they would instead "vote for Bernie, because it just might matter."