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A national political convention gives presidential candidates their first major opportunity of the campaign to connect with a national audience, reaching viewers by the millions and kicking off the race’s climactic final leg.

Until recently, it also usually meant the candidate would get a bump in the polls — like a guaranteed $200 for passing “Go” — though bravura performances tended to add a few extra points to the so-called convention bounce.

The average convention bounce has been on the wane in recent years — and with the coronavirus limiting the convention’s proceedings, this could be the year when the convention bounce disappears completely.

Of course, we probably won’t have enough polling results until next week to measure the effects of the Democratic National Convention, and we’ll have to wait longer to determine the impact of the Republican convention, which begins Monday. And, historically, convention bounces have mostly tended to wash out anyway: Both parties are about evenly likely to get a bump, and voters’ preferences tend to revert within a few weeks, or to be swayed in a fresh direction by newer developments.

Still, the fact that the campaign bounce itself is on the wane carries implications about the state of play in politics more generally.

Polarization and favorability

American politics have grown more deeply partisan over the past few decades, so there’s far less fluctuation in pre-election polling. Put simply, persuadable voters are in short supply.

So it makes sense that since 2004 the average convention bounce for candidates in both parties has been only around two points, according to data from the American Presidency Project compiled by FiveThirtyEight. That’s far less than the six-point average of the two previous cycles. Going back further, the average ticks up higher still.

With President Trump in office, polarization has only deepened: Public opinion of his performance has been markedly stable throughout his term, and most Americans report feeling strongly about him one way or the other, according to polls. Even among independents, his approval rating has hardly climbed higher than the low 40s.

“Historically the conventions have been a time for the party to unify,” Kyle Kondik, the managing editor of Sabato’s Crystal Ball at the University of Virginia Center for Politics, said in an interview. “But the bottom line is that there are not that many undecideds, and the conventions don’t really need to unify the party because the parties are already unified.” He pointed to an average that he had calculated using four high-quality national polls taken just before the D.N.C., showing that more than nine in 10 members of each major party said they would support their nominee.

Then again, it’s not all about the horse race. Polls show that supporters of Joseph R. Biden Jr. are overwhelmingly likely to say they’re casting their ballot mostly to oust Mr. Trump, not because they’re particularly eager to put Mr. Biden in the Oval Office. The Democratic nominee’s favorability rating currently runs about five points lower than the share of registered voters who say they plan to vote for him in the fall, according to averages from RealClearPolitics.

This isn’t exactly a fatal flaw — but it’s enough to cause Mr. Biden’s campaign some concern. If all the pro-Biden messaging at the convention failed to give him a bounce in head-to-head matchups against Trump, but nevertheless brought his favorability rating up, that could offer him a new sense of security.

Who’s watching?

Mr. Biden has remained something of a candidate-in-waiting since the pandemic began and he became the Democrats’ presumptive nominee. That’s partly by choice: Widely known as an uneven campaigner, the former vice president has seemed content to mostly stand aside as Mr. Trump publicly struggled to contain the coronavirus crisis or stem the economic downturn.

The convention was supposed to be Mr. Biden’s moment to turn on his campaign, addressing his largest audience yet after days of runway programming to boost his momentum. But then the pandemic got in the way.

The Democrats canceled their plans for a large in-person gathering in Milwaukee, dampening anticipation in the run-up to the event. This also significantly altered the vibe of the broadcast itself, making it feel less like a live event and, at times, more like a telethon.

Average live TV ratings over the four-night convention were down by almost 20 percent compared with 2016, according to Nielsen statistics (although those ratings don’t include online views, which likely rose this year). On some nights, the major broadcast networks saw a drop of nearly 50 percent in their convention viewership compared with four years ago.

The Republican Party will also hold a heavily altered version of its convention, though it will include an in-person nomination vote, for which state-party delegates will gather at the originally planned location in Charlotte, N.C.

Mr. Trump currently trails Mr. Biden by an average of eight points in national polls, according to various polling aggregators. If the Republican convention does manage to make a splash, Mr. Trump certainly has more room to grow than Mr. Biden did going into the Democratic convention.

“We have seen Trump weaker than he probably needs to be with seniors and even with white non-college voters,” Mr. Kondik said. “So I wonder if Trump can sort of bring some of those folks back with his convention.”

Michael Grynbaum contributed reporting.

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