A series of polls show President Donald Trump has improved his standing in several key Rust Belt states ahead of the 2020 presidential election.

Firehouse Strategies released surveys of Wisconsin, Michigan, and Pennsylvania on Tuesday indicating Trump is holding his own when matched against several of his potential Democrat opponents. These three states were key to President Trump’s victory in 2016. If Trump had lost each of them, he would not have had more than 270 electoral votes and would not have won the presidency. But since Trump flipped all three from Democrat control, along with other major wins in battlegrounds like Florida, Ohio, North Carolina, and Iowa, the president was able to defeat Democrat Hillary Clinton–winning a total of 306 electoral votes from 30 and a half states.

In the Firehouse Strategies survey of 535 Wisconsin likely voters, former Vice President Joe Biden leads Trump 46 percent to 40 percent. Although Trump trails Biden in Wisconsin in this poll, the Democrat frontrunner’s lead has shrunk by more than half since a similar survey of Wisconsin was conduced for Firehouse Strategies in March when Biden led Trump by 13 percent, 53 percent to 40 percent, in the Badger State.

Sen. Bernie Sanders (I-VT) was the only other candidate to lead Trump in the Wisconsin survey, out in front of the president 47 percent to 40 percent. Of the other two Democrats included in the survey, Sen. Elizabeth Warren (D-MA) was tied with Trump at 41 percent each, while South Bend, Indiana, Mayor Pete Buttigieg trailed the president 41 percent to 39 percent. This new Wisconsin survey was conducted from June 11 to June 13 and has a margin of error of 4.3 percent.

In 2016, Trump became the first Republican to carry Wisconsin at presidential level since Ronald Reagan’s landslide re-election in 1984. Even though Trump succeeded in flipping the state, it was by a narrow 23,000 vote margin.

Trump also appears to have made significant inroads within the state since the last presidential election. The poll found that Wisconsin’s Democrat “base,” meaning respondents voting Democratic regardless of matchup, was 32 percent. Trump’s base, meanwhile, was 37 percent.

The results were similar in Firehouse Strategies’ Pennsylvania survey of 565 likely voters with a margin of error of 4.2 percent. Firehouse Strategies found in a head-t0-head contest with Biden in the Keystone State, Trump was narrowly down, trailing the former Vice President by just one point–42 percent to 43 percent–well within the survey’s margin of error. When matched with Sanders, the president led 44 percent to 41 percent.

Against Buttigieg and Warren, Trump led each by more than 10 percentage points in Pennsylvania. Against Warren, Trump led 45 percent to 34 percent whereas against Buttigieg the president led 45 percent to 32 percent. This latest Pennsylvania survey was conducted from June 11 to June 13.

The results from Pennsylvania represent a significant shift in this firm’s findings in favor of Trump since March. Firehouse Strategies’ poll of Pennsylvania from March had seen Biden leading Trump by seven points in the state, 50 percent to 43 percent. That same poll in March had found Sanders and the president locked in a tie in Pennsylvania.

Not only have the Democrats diminished in standing in Pennsylvania according to this survey, but the poll found the party’s base in the state was only 24 percent, while Trump’s base was 42 percent. Trump carried Pennsylvania in 2016 by less than 45,000 votes, the first Republican to do so since 1988.

Despite the improvements in Wisconsin and Pennsylvania, Firehouse Strategies’ surveys found that Trump dropped slightly in Michigan. The Firehouse Strategies poll conducted from June 11 to June 13 of 587 likely voters in Michigan with a margin of error of 4.2 percent found that in a head-to-head matchup with Biden, Trump trails by three percentage points–43 percent to 46 percent. Even though Biden’s negligible lead over Trump is inside the poll’s margin of error, meaning he and the president are in a statistical tie in Michigan according to this survey, the same firm’s poll of Michiganders likely to vote back in March had seen Trump leading Biden by one percentage point in Michigan–also within the margin of error.

The president’s numbers look better against the rest of the Democrat field. In this latest survey in Michigan, Trump is tied with Sanders at 44 percent and he leads both Warren and Buttigieg in the state. Against Warren, the poll finds Trump leading 43 percent to 41 percent whereas against Buttigieg the president leads 44 percent to 40 percent.

Trump won Michigan by less than 11,000 votes in 2016. Democrats are optimistic about the state given the margin and their success in retaking the governorship and several down-ballot offices in 2018.

This poll, however, signals Michigan is likely to be a top battleground. The state’s Democrat base polled at 32 percent, while Trump’s base was 39 percent.

Although the race is still up for grabs in all three states, it is unclear if the push for impeachment by Congressional Democrats will only serve to bolster Trump’s chances.

Voters in each of the three states widely rejected the notion of impeaching Trump. When respondents were asked if they believed Congress should begin impeachment proceedings against the president on the basis of the Mueller Report: 57 percent were opposed in Wisconsin, compared to 56 percent in Pennsylvania, and 53 percent in Michigan.