Joe Biden's resurgence to become the Democrats' presumptive nominee is opening new paths to defeat President Trump, swing-state polls show.

The big picture: If Biden can keep his current leads over Trump in general-election matchups, it could create opportunities for pickups of three big states — Florida, Arizona and North Carolina.

That's without hurting Dems' chances to take back Michigan, Wisconsin and Pennsylvania, the key wins that sent Trump to the White House in 2016.

The Real Clear Politics national polling average shows that either Biden (+5.9%) or Sanders (+4.2%) would have been positioned to defeat Trump if the election were held today, but the role of the Electoral College means the results will come down to a handful of key states.

Sanders' endorsement of Biden on Monday shows a quick turn toward unity for Democrats. In 2016, it took Sanders until July to endorse Hillary Clinton.

It's still too early to know whether the coronavirus pandemic will make voters more or less likely to rally around Trump come November — or what the impact will be on voter turnout in those key states. A number of the surveys that comprise these polling averages were done pre-pandemic.

By the numbers: Of the states that held primaries before the pandemic paralyzed the country, Biden ran up margins in the places that will matter most in November.

Sanders lost to Biden in each of the primary states classified by Cook Political Report as a "Toss Up:" Florida (39 percentage points), North Carolina (30), Michigan (16.5) and Arizona (11).

Florida would be the big general election prize, with 29 electoral votes. Polls show it's the biggest reach for Biden — but it's also the most crucial for Trump to keep.

The RCP average puts the state at essentially a draw right now, with Biden leading Trump by 0.4 percentage points. In matchups against Sanders, Trump led by an average of 4.3 percentage points.

Included in that average is a recent, robust UNF poll of 3,244 registered voters conducted in the first week of April that gave Biden a 6-point advantage over the president.

Trump doubled down on his ties to the state by moving his official residency to Palm Beach in October.

Florida Gov. Ron DeSantis' slow coronavirus responses have the potential to hurt Trump in November if voters associate infections, deaths or economic pain more broadly with Republican leadership.

Between the lines: These state-by-state polls come during a period of relative popularity for Trump. Gallup surveys show his approval in the first few months of 2020 in the highest range of his presidency.

Yes, but: State polls have a shoddy track record in forecasting Trump's performance.

Clinton had a 55% chance of winning Florida in 2016, according to FiveThirtyEight's pre-election forecasts. She was also given more than a 75% chance to win each of Pennsylvania, Michigan and Wisconsin.

She lost all four.

What they're saying: A Trump campaign official argued that the polling averages don't reflect reality because a number of them incorporate surveys that are only as recent as February or early March.