Vice presidential contenders do not have a rich history of altering the course of an election. But Republican and Democratic political professionals alike believe Harris’ selection may carry more weight this year. That is because Biden, despite decades of government work, remains less sharply defined to voters than previous presidential nominees. And the rush to test the theory that Harris might matter is opening a critical new stage in the campaign.

“Look back four years ago, people had a pretty clear view of Hillary Clinton — it wasn’t always positive,” said Jeff Link, a veteran Democratic strategist who has studied voters who flipped from Barack Obama to Trump in 2016. “So, Tim Kaine’s attributes never rose to the level of their information about Hillary.”

But Biden is another story. Whole swaths of voters know little more about him than that he was Obama’s vice president, according to Link and other Democrats who have conducted focus groups in recent months. And Biden, who will turn 78 in November, has referred to himself as a “transition candidate,” putting more weight on Harris, who ran for president last year and is widely expected to run again.

For voters, said Link, the vice presidential selection “will help define Biden in many ways.”

In picking Harris, Biden went with the vice presidential prospect widely regarded as his safest choice. The animus toward Trump on the left enabled Biden to bring progressive Democrats into his campaign largely on his own terms — it did not require a more liberal running mate, such as Sen. Elizabeth Warren or Rep. Karen Bass. Protests surrounding the death of George Floyd heightened the imperative to select a Black woman.

The upsides of Harris were obvious. An experienced, center-left politician, her presence on the ticket will serve to reinforce Biden’s own moderate credentials — and, after she lacerated him in a debate last year, add dimension to his reputation for empathy and consensus-seeking. At 55, the California senator balances Biden’s advanced age with youth. A historic selection who would become the first Black woman vice president if elected, she is a capable debater who Democrats expect will excoriate Trump in her debate with Vice President Mike Pence. And she will raise money.

But any change in the direction of the campaign is a source of uneasiness for Democrats. The phase of the election that saw very little of Biden — mostly from his house, where he has been secluded because of the coronavirus — was going well for him. Nothing that Trump threw at Biden appeared to stick, and Biden has been running ahead consistently in national and most swing state polls.

Now, with Trump’s window for turning his campaign around rapidly closing, Republicans at least see an opening, hopeful that “Biden-Harris” may prove weaker than Biden alone.

Within minutes of the selection, the Trump campaign released a statement depicting Harris as “phony Kamala” — a striver who would overlook her past criticism of Biden and her own record as a prosecutor “to appease the anti-police extremists” on the left.

Roe said, “He just nominated a radical from Oakland, California … a candidate from the left coast who has signed on to every single big government, open borders and high taxes initiative since she has been elected.” And the Republican National Committee blared in an email, “Far-Left Radical Harris Gives Democrats The Most Extreme Ticket In History.”

“Nothing’s sticking to Joe, right?” said Mike Madrid, a Republican strategist and one of the founders of The Lincoln Project, a Republican group opposing Trump. “So, they’re going to have to find a black woman they can hang something on, and just drive, drive, drive it.”

He said, “I don’t know if they’ll be able to handle it competently … but they’re trying to get a grip on something, they’re asking for something to get some traction, and race has always been their fallback.”

There are old avenues for Republicans to exploit. There was Larry Wallace, a top aide to Harris who resigned his position in late 2018, amid accusations of harassment during his time at the California Department of Justice. Harris has said she was unaware of the allegations against him.

And there is the standard attack on Harris' association with Willie Brown, who Harris once dated and who, as a speaker of the California Assembly, named Harris to well-paid positions on the California Medical Assistance Commission and Unemployment Insurance Appeals Board. Democrats are bracing for Republicans to depict her and Biden as a pair of careerists.

Last week on Fox News, Tucker Carlson appeared to set the stakes. Describing the eventual vice presidential nominee as “the actual Democratic nominee — that’s the person who will run the government if Biden wins in November," he called Harris “so transparently transactional that even Democratic primary voters, who have a strong stomach, found her repulsive.” Then, recalling the misogynistic tropes of likability from the 2016 campaign, Carlson added that “pretty much no one who knows Kamala Harris likes her.”

The tactical obstacle for Republicans is that they are unlikely to unearth anything disqualifying in Harris’ past, after several statewide campaigns and a presidential primary in which she faced intense scrutiny of her record — as well as stiff criticism from progressives for her record as a prosecutor.

And so far, the vice presidential roll-out has been successful for Biden. More than any time since Biden secured the Democratic nomination five months ago, he has been generating significant public attention. An unusually long, public vetting of vice presidential prospects allowed him to float a range of prospective candidates — and to associate himself with a roster of qualified women, appearing with many of them in virtual events.

The Democratic National Convention next week will give him an opportunity to further illuminate his candidacy and the direction of the party on his watch.

“This thing is a referendum on Trump and COVID,” said Pete Giangreco, a Democratic strategist who has worked on nine presidential campaigns. “And as long as the VP says that 10 to 20 times a day from now until November, it will all be good.”