Their mid-September survey  didn’t offer much hope that Trump could hold the heartland:

Overall, Trump continues to struggle in these three states. He trails Biden in all three states, and only leads Warren and Sanders in Michigan. His approval rating is also underwater by 4 points in Michigan, 9 points in Pennsylvania, and 12 points in Wisconsin. There seems to be more upside for the Democrats than Trump at the moment, though with over a year left, the landscape can change drastically between now and election day, as it did in 2016. …

In Wisconsin, we find all three candidates with a lead over Trump, though still within the margin of error. Sanders has the largest lead at +6, followed by Biden at +2 and Warren at +1. Among independents/others, Sanders leads Trump by 9 points. Warren trails Trump by 1 among this group, and Biden leads by only 2.

In Michigan, Biden fares the best among the three candidates, the only one of the three with a lead (+4), though also within the margin of error. We find Warren and Sanders both trailing Trump, by 2 and 3 points respectively. Interestingly, the gender gap is larger with a Biden-Trump matchup than with a Warren-Trump matchup. Biden leads by 14 points among females and Trump leads by 12 among males, while Warren leads by 8 among females and Trump leads by 12 among males. Sanders leads females by 6, while Trump leads by 13 among males in that matchup.

In Pennsylvania, President Trump is currently losing in head-to-head matchups against each of the three Democratic frontrunners. Trump performs the worst against Biden, where the Scranton-native enjoys a lead of 4 percentage points. Against Warren and Sanders, Trump is down 2 percentage points to each.

The timing of this turned out to be propitious. These surveys took place between September 7-9, which was just days before news broke of the Ukraine-Gate whistleblower and the start of the latest version of impeachment. Almost since the moment these surveys ended, Trump and House Democrats have locked horns over their efforts to amplify the scandal into his removal from office.

Has that worked? According to their latest survey, it hasn’t — and it might have backfired.  Three months later, Trump has broken free of the Democrats in all three states, and against all five of their top contenders — including Joe Biden:

As the impeachment process heats up in Washington, Donald Trump is seeing a boost in support in crucial swing states. Across the board, President Trump is polling well against the Democratic field in each of these battleground states. Notably, Vice President Biden has seen a sharp decline in support in our surveys as he currently runs behind President Trump in each of the three states.

As the race currently stands, President Trump is in the lead in Michigan, Pennsylvania, and Wisconsin in hypothetical match-ups against former Vice President Joe Biden, Senator Elizabeth Warren, Senator Bernie Sanders, Mayor Pete Buttigieg, and former Mayor Michael Bloomberg. Across the three states, Trump’s closest contest is against Joe Biden, although the president leads by an average of 6 percentage points against each Democrat.

The turnaround was significant in each state but most dramatic in Wisconsin:

Wisconsin was President Trump’s best state in this round of battleground polling. Against almost each of his Democratic challengers, President Trump held a double-digit lead. The president is currently running up nine percentage points against Biden, twelve points against Warren, thirteen points against Sanders, eleven points against Buttigieg, and twelve points against Bloomberg. Among non-partisan voters, Wisconsinites favored President Trump by double digits. For example, against Senator Elizabeth Warren, Trump led by over 18 percentage points among non-partisans.

Firehouse has a number of graphics to explain the results of their surveys, but this one perhaps captures the sudden slide best:

This demonstrates a couple of points to keep in mind, especially when considering the GOP affiliation of the source. First, Firehouse has had Trump in trouble all year until this round of polling, and not just about Biden. Second, while this is clearly a reflection of Biden as a candidate too, the trend isn’t just about Biden. Biden’s still the most competitive opponent against Trump in Michigan and Wisconsin, and second-most in Pennsylvania behind Mike Bloomberg.

In fact, it’s probably easiest to conclude that this isn’t really about the candidates at all. It’s not even really about Trump per se. It’s about impeachment and voter resentment over it, which comes across pretty strong in the other questions on the survey:

In each of these battleground states, we find that a majority of likely 2020 voters do not support the impeachment and removal of President Trump from office. Impeachment and removal is opposed by 50.8% of voters in Michigan, 52.2% of voters in Pennsylvania, and 57.9% of voters in Wisconsin. Non-partisan voters in Michigan (70%) and Wisconsin (61%) oppose impeachment and removal while non-partisan voters in Pennsylvania slightly support it (46.4% to 40.9%). When asked about whether congressional Democrats should be spending their time impeaching Trump or focusing on policy issues, a majority of these battleground state voters choose “focus on policy issues” (MI: 59.4%; PA: 63%; WI: 67.2%).

With that in mind, the danger here is that the race reverts to previous form once impeachment ends. It’s also not unusual for incumbents to lead polling while the opposition party fragments between its primary options. In fact, that’s why the earlier polling by Firehouse and others should have had Republicans very concerned. In either case, this poll may not reflect a long-term trend in the presidential race, not unless Democrats are foolish enough to keep impeachment going all through 2020.