President Trump and Joe Biden begin the general election campaign locked in a highly competitive contest that remains fought along the lines of the 2016 presidential election, according to national and battleground state polls.

If anyone holds the early edge, it is Mr. Biden. He leads by an average of six points in national live-interview polls of registered voters. But the election will be decided by voters in the battleground states, not registered voters nationwide, and there the story is not nearly so clear or rosy for Mr. Biden.

At the moment, a reasonable estimate is that Mr. Biden is performing four or five points worse among likely voters in the critical states than he is among registered voters nationwide. As a result, he holds only a narrow and tenuous edge in the race for the Electoral College, if he holds one at all.

Even under ordinary circumstances, with seven months to go until the election, there would be plenty of time for the race to change. This cycle, the country also faces a pandemic and a severe economic downturn with the potential to upend the race.

Already, an initial uptick in the president’s approval rating has dissipated, perhaps because a rallying effect has given way to more focus on the administration’s coronavirus response. There will be many opportunities for the polls to shift again, and the president faces many downside risks without a return to normal life and to economic growth before the election.

But at least for now, the polls suggest that American voters are divided along familiar lines, despite countless events that seemed to have the potential to redraw them.

The president begins the campaign with strong support from the white working class who powered his upset win four years ago. He leads among white voters without a college degree, 61 percent to 32 percent, in an average of live-interview polls conducted since March 15, matching or perhaps even exceeding his margin over Hillary Clinton in methodologically similar polls conducted late in the 2016 campaign.

The results suggest that Mr. Biden, despite his reputed appeal to blue-collar workers, has made little to no progress in winning back the white voters without a college degree who supported Barack Obama in 2012 but swung to Mr. Trump in 2016.

Mr. Biden counters with a wide lead among white voters with a college degree, who support him by a similar or greater margin than they did Mrs. Clinton four years ago. Over all, he leads, 55-40, among that group of registered voters in an average of recent surveys.

Mr. Biden also holds the expected wide advantage among nonwhite voters, though here there is consistent evidence of a small yet discernible shift in the president’s direction, including in the large series of New York Times/Siena College polls from last November. Mr. Trump, in contrast, seems to do a tick worse among white voters than he did four years ago, whether because of a slight decrease in his standing among college-educated white voters or the growing share of white voters who possess a four-year degree.

As a result, Mr. Trump appears to retain his relative advantage in the disproportionately white working-class battleground states that decided the 2016 presidential election. Mr. Biden leads in polls of registered voters in these states, but by a narrower margin than he leads nationwide. It raises the possibility that Democrats could win the most votes and lose the White House for the third time in six presidential elections.

Wisconsin, the tipping-point state in 2016, has backed Mr. Biden by just over one percentage point in an average of seven live-interview surveys conducted so far this year, compared with a six-point Biden lead in national surveys over the same period.

Mr. Trump’s relative advantage in the Electoral College, though persistent so far, is not unshakable. To this point, his apparent strength in Wisconsin has tended to expand his Electoral College advantage. But a fairly modest shift in that state or another could expand or contract it.

Arizona and Florida are two possible candidates. White voters without a degree represent a smaller share of the electorate in those states than they do in the Midwest. Arizona polls in particular give Democrats some cause for hope: Mr. Biden has led live-interview polls by four points in Arizona. A breakthrough by Democrats in Arizona would essentially cancel out their weakness in Wisconsin. It would make Pennsylvania, where Mr. Biden leads by about two points, the pivotal state on the electoral map.

One reason for Mr. Biden’s strength in Arizona and Florida might be older voters, who represent an above-average share of white voters in the two states. On average, Mr. Biden leads among voters over age 65 by a margin of 53 percent to 44 percent nationwide, including a lead in every live-interview national poll reporting a result for the group. It is a substantial improvement over Mrs. Clinton’s six-point deficit among the group in pre-election polls in 2016.

Mr. Biden’s early strength among older voters is not easy to explain. It cannot be fully accounted for by the changing composition of each age group, although the ascent of the baby boomers into the oldest age cohort may be part of the reason, along with the gradual departure of the more conservative Silent Generation from the electorate altogether. Mr. Trump seems to have made gains among voters 45 to 65, or perhaps even younger, canceling out his losses among older voters over all.

Mr. Biden’s relative strength among older voters may also help counteract his expected weakness among likely voters, relative to registered voters, when pollsters apply likely-voter screens later in the cycle. In polling, Republicans usually fare about two points better among likely voters than registered voters, who tend to be relatively young and diverse.

This cycle, the coronavirus pandemic raises additional questions about the eventual turnout, particularly if it leads to widespread voting by mail. But no matter the method, Democrats typically find themselves at a turnout disadvantage, and it is doubtful that Mr. Biden will maintain the whole of his current polling advantage among likely voters. Even when Democrats benefited from a surge in turnout in well-educated suburbs during the 2018 midterm elections, Republicans fared better among likely voters than among registered voters.

Together, Mr. Trump’s relative advantage of one to two points among likely voters compared with registered voters — and his relative advantage of three and even four points in the tipping-point states — means that the typical national poll of registered voters is probably around four or five points worse for Mr. Trump than his standing among likely voters in the most pivotal states. Mr. Biden’s already narrow polling lead in states like Wisconsin, Pennsylvania or Arizona might be vanishingly small after a likely voter screen.

Of course, no one knows what American life will look like by the time of the election. Perhaps the country will still be in lockdown, saddled by 30 percent unemployment, and convinced that the president’s slow response cost lives and damaged the economy. Or maybe the country will be swept by euphoria as lockdowns are lifted a month or two ahead of the election and a liberated population sends its children to school, visits friends, goes to the park and enjoys double-digit G.D.P. growth in the third quarter.

The pandemic also could change how the election is administered, potentially yielding a novel turnout that’s impossible to predict at this stage. It undoubtedly has the potential to reshape the views of the electorate, even if it hasn’t done so yet.