Joe Biden is holding on to his sizable lead over President Donald Trump in the national polls, but there are reasons for the Democratic nominee to worry about the handful of key states that could ultimately decide the election.With just 19 days left until the Nov. 3 election, Biden’s position on a national level looks strong, according to major polling trackers.Here’s what they showed Thursday morning:The former vice president has maintained a consistent lead by those measures, despite the tumultuous, and in many ways unprecedented, state of American politics. One of the biggest shifts of the race occurred within the last month, when Biden’s polling lead grew wider after Trump’s widely panned debate performance and the revelation that he had contracted the coronavirus.Read more: Democrats are crushing Republicans in the fundraising raceYet Biden holds a narrower advantage in states that will likely decide an Electoral College victory.RealClearPolitics’ gauge of polls in six crucial battleground states — Florida, Pennsylvania, Michigan, Wisconsin, Arizona and North Carolina — currently shows Biden with a 4.9-point lead over Trump.Those same states showed a larger gap in the polls for former Democratic nominee Hillary Clinton at this point in the 2016 race against Trump, which she lost. On Oct. 15, 2016, Clinton was 5.4 percentage points over Trump in those key swing states, according to RealClearPolitics.Republicans are also outpacing Democrats in voter registration in swing states. In North Carolina, for example, Democrats lead Republicans in registered voters by a difference of nearly 400,000. That’s down from the same reporting period in 2016, when Democrats had about 645,000 more voters registered in the state.Clinton was widely favored to win the 2016 election. While she walked away with nearly 3 million more votes overall than her Republican rival, a series of narrow upsets in rust-belt swing states ultimately helped Trump net 306 electoral votes, leaving 232 for Clinton.There are other factors at play this time around.Both major party candidates in 2016 were historically unpopular, and voter turnout was historically low. That was a huge benefit to Trump, whose rock-solid base of supporters helped him overcome low favorability ratings nationally.In 2020, fewer voters say they strongly dislike Biden, while Trump’s unpopularity has been fairly consistent throughout his first term in office. Crucially for Biden, that could pay dividends in the form of turnout among independents, who support the former vice president more than the incumbent president.Biden is also raising monstrous amounts of money in the final leg of the race — and he’s focusing his spending on swing states. The campaign has spent more than $500 million on TV, digital and radio ads so far this year, according to ad tracking firm Advertising Analytics. Trump’s campaign, meanwhile, has gone dark in certain states and shifted TV ad spending toward Florida, Georgia, Pennsylvania and North Carolina. He has, however, outspent Biden on Facebook ads in several key states.The coronavirus pandemic has changed the race, as well. The Covid-19 crisis, which has killed more than 216,000 people in the U.S., is not only an election-defining political issue but a catalyst for a massive change in the way Americans are voting.More than 10.6 million voters have already cast their ballots in the November election as of Tuesday morning, according to data from the U.S. Elections Project — more than 10 times higher than at roughly that point in 2016. The expected flood of mail-in ballots could increase turnout among different demographics, but concerns have grown over the U.S. Postal Service’s ability to promptly handle the influx of election mail.There’s another confounding variable, which may not even be revealed yet: a potential October surprise.Voters have learned to expect the unexpected in the month before a presidential election. FiveThirtyEight’s Nate Silver has said that a late-October letter from then-FBI Director James Comey, which announced that the FBI had found “pertinent” emails on Clinton’s private email server, likely cost the Democrat the election.A New York Post report published Wednesday, which claims to show “smoking gun” emails related to Biden and his son, Hunter Biden, has already caused a stir.The report has yet to be matched by other news outlets, and many journalists have expressed strong skepticism about the story. It alleges Hunter Biden attempted to set up a meeting between his father and a top executive at a Ukrainian company he worked for while Joe Biden was vice president.It’s unclear whether the Post’s report will have a noticeable impact on the race. But the president and his supporters have railed against the decisions by Facebook and Twitter to limit distribution of the story on their platforms, accusing the tech giants of politically motivated censorship.Biden’s son was at the center of Trump’s impeachment fight, which revolved in large part around a phone call in which Trump asked Ukraine President Volodymyr Zelenskiy to “look into” the Bidens. Trump was impeached in the House and acquitted in the Senate.Biden campaign spokesman Andrew Bates pushed back hard on the Post’s report.”Investigations by the press, during impeachment, and even by two Republican-led Senate committees whose work was decried as ‘not legitimate’ and political by a GOP colleague have all reached the same conclusion: that Joe Biden carried out official U.S. policy toward Ukraine and engaged in no wrongdoing. Trump Administration officials have attested to these facts under oath,” Bates said.”The New York Post never asked the Biden campaign about the critical elements of this story,” he added. “They certainly never raised that Rudy Giuliani — whose discredited conspiracy theories and alliance with figures connected to Russian intelligence have been widely reported — claimed to have such materials.””Moreover, we have reviewed Joe Biden’s official schedules from the time and no meeting, as alleged by the New York Post, ever took place,” Bates said.