The Democrats began their quadrennial convention buoyed by polls that show the Biden-Harris ticket with a significant lead over President Donald Trump. But some of the polls also point up a key problem: Voters are less for former Vice President Joe Biden than they are against Trump.

All elections are about turnout in the toss-up states, as Hillary Clinton and the Democrats learned four years ago. Former First Lady Michelle Obama reinforced that lesson during her convention speech Monday night after noting that President Trump and his supporters, "who know they cannot win fair and square at the ballot box, are doing everything they can to stop us from voting."

"This is not the time to withhold our votes in protest or play games with candidates who have no chance of winning," Obama said. "We have got to vote like we did in 2008 and 2012. We've got to show up with the same level of passion and hope for Joe Biden."

But with balloting beginning in six weeks in some states, is there enough time to create passion? Or will the passion against Trump be sufficient to drive him from power?

A Pew Research Center poll released last week found high levels of engagement among voters — unusual for a presidential election with an incumbent — but also high levels of concern by people about whether they will actually be able to vote in the face of the pandemic.

The poll was taken before news reports about the U.S. Postal Service warning state election officials that it might not be able to deliver mail-in ballots on time, adding to perceptions that the Trump administration is trying to undermine the election itself.

The worrisome part of the poll, though, is what it revealed about relative passion among Biden supporters.

"For Biden supporters, opposition to Trump is by far the most frequently mentioned reason why they support him," Pew said. "Asked an open-ended question about the main reason they support or lean toward Biden, a 56% majority of his supporters cite their opposition to Trump; far fewer mention Biden's leadership or performance as a candidate (19%) or his personality (13%)."

Among Trump supporters, only 19% listed opposition to Biden as their prime reason for supporting the president's reelection. But only 23% cited Trump's leadership and job performance. His core voters are loyal for reasons beyond easy poll measurements.

"This is very different from the 2016 presidential campaign," Pew notes, "when opposition to the other candidate was among the top reasons given by supporters of both Trump and Hillary Clinton for their voting decisions."

Anger can be a strong motivator for voters, but it burns out. And after more than four years of listening to Trump's stream of dark consciousness, we have to wonder how many Americans have become inured. As former presidential candidate Sen. Bernie Sanders noted in his speech Monday, "the unthinkable has become normal."

Presidential nominating conventions usually are centered on buffing up the nominee's image and launching key party activists into the final weeks with fire and a sense of purpose. And Monday's opening two hours did a pretty good job of doing that for Democrats, particularly the segment about Biden and his days commuting from Delaware via Amtrak to Washington, D.C., when he served in the Senate. Biden has long enjoyed a reputation as being genuinely genial, and that video segment spotlighted that endearing part of his personality.

The tag team of Sanders, who delivered a stark and forceful message to his followers to fall in line because "the future of our democracy is at stake," and Obama went on to provide an effective motivational segment to the party activists.

But activists are the folks who rarely sit out an election. It was not their disappearance four years ago that let Trump win in Wisconsin, Michigan, Pennsylvania and other close states, even as he lost the national (and irrelevant) vote.

That's where the fight must be waged this time around — in the hearts and minds of those whose past political indifference helped land us in the mess we're in today. And the calendar is running out.