Surging Democratic front-runner Bernie Sanders is leading the party’s primary contests in Pennsylvania, Michigan, and Wisconsin, a new poll finds, while President Donald Trump is running neck-and-neck with almost all of the Democratic contenders in those critical battleground states.

The poll, conducted before Sanders’ victory in the Nevada caucuses Saturday, suggests that only marginal differences separate the Democratic candidates when it comes to their strength against Trump. And it portends another general election that could be decided by a handful of votes in any one of those swing states. The poll found Trump trailing all the Democratic candidates in Michigan, but virtually tied with each of them in Pennsylvania and Wisconsin.

If the rest of the electoral map stays the same as in 2016, Democrats would need to win all three states to defeat Trump.

“All three states are up for grabs in 2020,” said Barry Burden, director of the University of Wisconsin-Madison’s Elections Research Center, which conducted the poll. “Trump is in a more difficult position in Michigan than the other two states, but each of the Midwest battlegrounds could be won by either party, almost regardless of who becomes the Democratic nominee.”

The poll provides the second recent look at the states that, by tiny margins, sealed the 2016 election for Trump, after decades of voting Democratic in presidential races. A Quinnipiac University poll released Thursday also found Trump trailing the Democrats in Pennsylvania and Michigan, but ahead in Wisconsin.

The states’ impact on the Democratic primary is less clear, because Wisconsin and Pennsylvania will vote relatively late in the process. But given the Keystone State’s size and its cache of delegates, it could play a major role in the primary if the race continues without a dominant leader.

Pennsylvania primary: Sanders leads, Bloomberg rises

Sanders, a Vermont senator who has had the strongest start in the primary, has support from 25% of Pennsylvania Democrats, while former Vice President Joe Biden is second, with 20%. Mike Bloomberg, the billionaire former New York mayor, is on his heels at 19%.

Further back are Pete Buttigieg (12%), Sen. Elizabeth Warren (9%), and Sen. Amy Klobuchar (5%).

That makes for the tightest Democratic race in any of the three battlegrounds, though with Pennsylvania voting April 28, there’s also more time for shifts.

In Michigan, which will vote March 10, Sanders has 25% support among Democrats and Biden is next, with 16%.

Sanders has his biggest lead in Wisconsin, where he has 30% support. Biden, Bloomberg, Buttigieg and Warren all have 12% or 13% there. Wisconsin will hold its primary April 7.

Pennsylvania general election: A dead heat

Decided by just 44,000 votes in 2016, Pennsylvania is looking at another narrow result in the general election, the University Wisconsin poll suggests.

Sanders fares best against Trump in a hypothetical match-up there, leading 47% to 45% — within the poll’s 3% to 4% margin of error. Biden leads Trump 46% to 45%, and every other Democrat is even with Trump in the Keystone State.

About 45% of Pennsylvanians strongly or somewhat approve of Trump’s job performance, and 52% somewhat or strongly disapprove. Those numbers were similar in Michigan and Wisconsin.

Democrats lead Michigan, tight in Wisconsin

Every Democrat leads Trump by 1 or 2 percentage points in Wisconsin, this poll found (a stark contrast from Quinnipiac’s result). But that’s a statistically insignificant lead.

Democrats fare better in Michigan. Sanders leads Trump 48% to 41%, Buttigieg has a 6 percentage point edge on the president, and the rest of the Democrats lead by 3 to 4 percentage points.

Trump won each of the three states by less than 1 percentage point in 2016.

About 90% of voters who supported Trump in 2016 plan to vote for him again, the poll found. But those who didn’t vote in 2016 favor Democrats by double-digit margins.

Democratic differences

The Democratic candidates show differing strengths.

Sanders dominates with those aged 18 to 29, garnering 55% of their support. He has less than 9% support among those older than 65. Bloomberg narrowly leads that age group.

Union members in the three states prefer Biden. He and Warren have more support in urban areas, while Buttigieg does best in small towns and rural regions.

The poll was conducted Feb. 11-20, in the immediate aftermath of Sanders’ win in the New Hampshire primary. The poll surveyed 1,300 adults in Michigan, 1,300 in Pennsylvania, and 1,000 in Wisconsin. The analyses of the primary and general elections were based only on the responses of registered voters.

The margin of error was 3 to 4 percentage points for full state results, though larger for smaller samples, such as the Democratic primary.