Early swing state polls suggest President Trump will have his work cut out for him to win reelection to a second term.

Trump's upset victories in Pennsylvania, Michigan and Wisconsin propelled him to the White House in 2016. He was the first Republican presidential candidate to win any of those states since 1988.

Today, most polls show Trump behind former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE — the current front-runner in the Democratic race — in all of those swing states.

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That's a problem, because if the 2016 map otherwise stays the same, a Democratic candidate who wins those three states will win the Electoral College.

“Wisconsin seems to be a real problem, the 2018 midterms were a disaster for Republicans there, and Biden seems to have unique appeal in Pennsylvania,” said Alex Conant, a partner at the GOP firm Firehouse Strategies.

“Trump appears to be running strongest in Michigan, and he really only needs to hold on to one of these to win reelection. But it’s going to be very competitive and Trump is going to need to spend a lot of time in those three states and hope for the kind of turnout we saw in 2016,” he said.

Worse for Trump, polls also show him trailing Biden and Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.) in a number of other swing states.

A poll of North Carolina released this week showed Biden up 12 points on Trump and Sanders leading him by 8 points. Former President Obama is the only recent Democratic presidential candidate to win the Tar Heel State, with the previous Democratic victory there dating back to 1976.

Meanwhile, a trio of states that Trump won easily in 2016 – Texas, Arizona and Iowa – appear to be up for grabs, with some surveys finding Biden in the lead in all three states. A survey from Quinnipiac University Poll released Wednesday found Biden leading Trump 48 percent to 44 percent in Texas, with the other Democrats not far behind.

It's very early in the race, and the Trump campaign believes a stronger economy will help him win another four years in office. They also note that the president was repeatedly counted out in the 2016 race, with pundits pointing to polls that discounted the Republican's chances.

There's also no guarantee that Biden will emerge from a tough Democratic primary as the party's nominee.

That was evidenced by the blowback this week after he said he still supports a ban prohibiting the use of federal funds for certain abortion services and after his campaign said it inadvertently lifted some lines from other groups when unveiling his climate change plan.

Experts also believe Trump could post gains once the Democratic challenger starts taking hits under the full weight of a general election campaign.

But the early polls do have some warning signs for Trump and the Republicans, particularly in the Rust Belt, if Biden or Sanders emerges as the Democratic nominee.

Polling released last month by Priorities USA, the largest Democratic super PAC, found a generic Democratic candidate leading Trump in Wisconsin, Michigan, Pennsylvania and Florida by 9 points overall.

Priorities USA said that if the election were held today, the three Rust Belt states would flip back to the Democratic column, giving the party’s nominee a 279-259 victory in the Electoral College.

In Michigan, which Trump won by just over 10,000 votes in 2016, the polls appear to be moving against the president.

In a March survey, Firehouse-Optimus had Trump running neck and neck with Biden and Sanders, leading both by a slim 46-45 margin.

But surveys released since then have found Trump trailing in the Wolverine State, with a Glengariff Group public opinion poll released Tuesday by The Detroit News finding Biden and Sanders over the 50 percent mark and leading Trump by 12 points.

That survey also found South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegThe Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice Hillicon Valley: FBI, DHS warn that foreign hackers will likely spread disinformation around election results | Social media platforms put muscle into National Voter Registration Day | Trump to meet with Republican state officials on tech liability shield MORE and Sens. Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (Mass.) and Kamala Harris Kamala HarrisHarris faces pivotal moment with Supreme Court battle Nearly 40 Democratic senators call for climate change questions in debates Joe Biden has long forgotten North Carolina: Today's visit is too late MORE (Calif.) with modest leads in head-to-head match-ups with the president.

An Emerson College survey found Biden ahead by 8 points in Michigan also revealed that 52 percent of respondents there said they are very unlikely to vote for Trump, compared with 37 percent who said they are very likely to vote for him.

Meanwhile, Democrats won sweeping victories across Wisconsin in 2018, led by Gov. Tony Evers, who ousted former Gov. Scott Walker (R).

Nearly every early poll of the Badger State has found the top Democratic contenders with a lead over Trump, who carried Wisconsin by fewer than 25,000 votes over Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE in 2016.

An Emerson College survey found Biden leading Trump 54-46. A Firehouse-Optimus survey put Biden up by 13 points and Sanders up by 7. Zogby Analytics has Biden up by 10 points. The Republican polling outlet WPA Intelligence is the only firm to find Trump with a lead over Biden, showing him with a 46-42 advantage.

Trump is perhaps polling the weakest in Pennsylvania.

An Emerson College survey finds Biden and Sanders each leading Trump by 10 points in the Keystone State.

The latest survey from Quinnipiac University Poll found Biden with an 11-point lead in Pennsylvania, boosted by a 14-point advantage among independents, a 24-point lead among women and a 43-point advantage among nonwhite voters.

The economy seems as if it should be giving Trump a boost in Pennsylvania. Fifty-four percent of voters in the state said they’re better off financially than they were in 2016 and 71 percent described the economy as “excellent” or “good.”

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But GOP pollster David Winston said Trump’s tariffs have injected volatility into the markets and have negatively impacted voters’ otherwise rosy views of the economy.

“There’s a sense of uncertainty,” Winston said. “Trump needs to reestablish a clear sense that the economy is headed in the right direction and until he does, these surveys will be soft for him and I suspect that’s what we’re seeing in these swing state polls.”

Several other problem spots have materialized for Trump across the map, underscored by his surprising polling weakness in Texas and North Carolina.

The president carried Iowa by nearly 10 points in 2016, but an Emerson poll from March found Biden ahead by 6 points.

And Trump won Arizona by 3 1/2 percentage points in 2016, but an OH Predictive Insights survey found Biden leading there by 5 points.

Republicans aren’t sounding the alarm yet, believing their voters will come home for Trump when it matters the most. The Trump campaign is also hunting for alternative pathways to victory, eyeing Nevada, where polls are tight, as well as Minnesota, New Hampshire and New Mexico as having potential to swing Republican.

“We are hundreds of news cycles, and an entire Democratic primary process away from a head-to-head poll mattering,” said Robert Blizzard, a GOP pollster. “There were public polls that showed Trump losing those states as late as the fall of 2016 and he won them. Horse race is fun and interesting but meaningless until the end.”

--This report was updated on June 6 at 4:57 a.m.