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The speculation drives headlines for months, with reporters tracking private flights and parsing every statement for the slightest inkling as to whom a candidate might select as a running mate. Meanwhile, the campaign carefully orchestrates a rollout — and tries to prevent leaks.

On Tuesday, Joseph R. Biden Jr. announced that Senator Kamala Harris of California would be his running mate, blasting out a text message to supporters and a tweet in the late afternoon. The Biden website was updated within seconds, and an hour later, the campaign sent its first fund-raising email under the new “Biden-Harris” logo.

It was the culmination of months of work from the campaign’s digital team, who, like the reporters chasing the story, had largely been working with little more information than the public had.

The online rollout of a running mate begins long before the presidential candidate has even decided on a partner. It’s a perfect opportunity for the campaign to funnel the intense public interest in the selection into a list-building operation for emails and text messaging.

The Biden campaign had been directing supporters to sign up for its texting list with the promise of being “one of the first” to know (perhaps taking a lesson from the 2008 Obama campaign, which also promised to announce the pick first through a text message — though Mr. Biden’s selection was reported beforehand in the news media.)

“This is the third time the Democratic nominee has promised to share the info first with text message subscribers,” said Jenna Lowenstein, the former digital director for Hillary Clinton’s campaign in 2016 and deputy campaign manager for Senator Cory Booker in 2020. “Texts we think of as a smaller tool than email or advertising, but it’s really emerging as this powerful platform for fund-raising and organizing.”

The campaign’s digital staff members are often as much in the dark about the pick as the political reporters scrambling to break the news. Much like reporters who draft multiple versions of stories to have them ready to publish as soon as the news breaks, the digital team prepares for multiple scenarios drawn from a short list of candidates.

That means multiple websites with different graphics, multiple introductory videos and multiple initial fund-raising emails.

“In our case, we prepared for a short list of four people internally,” Ms. Lowenstein said of the 2016 Clinton effort. “We printed four versions of signs. Our design team did four versions of the logo. We did four videos. We did four of everything.”

After the announcement, the political battle to define the new pick begins nearly instantaneously online. Indeed, the Trump team was ready with a polished attack ad against Ms. Harris, labeling her as a member of the “radical left.” The Biden campaign sent out a lengthy fund-raising email from Mr. Biden, explaining in personal terms why he had selected Ms. Harris and calling her “smart, tough, and ready to lead.”

“You have to focus on what are the three things you want someone to know about this person,” said Ms. Lowenstein, who said that for Senator Tim Kaine, Mrs. Clinton’s choice, it had been his commitment to middle-class families, his Spanish skills and his history of service. “We had this concrete list of things, because the public isn’t going to absorb 20 facts about this guy,” she said.

Finally, there is the transfer of digital assets. Most vice-presidential contenders will have their own websites, their own social media handles, their own email lists. These now must all be handed over to the campaign, for at least the remainder of the race, so it can leverage the running mate’s network.

“How can fast are you going to get that to be a donate page for the campaign; how fast can you get them branded assets?” Ms. Lowenstein said. “Is there anything up there that needs to be not up there? And often these vice-presidential candidates have really small teams — they don’t have a huge digital army that can switch a website and domain in five minutes. You have to get on the phone with people and get them to hand you keys really quickly.”

Within minutes of today’s announcement, kamalaharris.com redirected to joebiden.com.

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Ad of the week: New (old) messages for Trump

Right before the Trump campaign paused its advertising strategy in late July, it was running an aggressive ad campaign inaccurately tying Joe Biden to the movement to defund the police.

But since returning to the airwaves for the past week, the campaign has reverted to more traditional messages for President Trump. It’s a more disciplined approach, using themes he has employed since his political ascent in 2016.

The message: The campaign’s new ad uses two edited quotes from Mr. Biden to drive home two lines of attack against the former vice president, on taxes and illegal immigration. The first clip is selectively edited to show a response Mr. Biden gave to a voter who talked about benefiting financially from the Trump tax cuts. “If you elect me, your taxes are going to be raised, not cut,” Mr. Biden says to the voter. The ad portrays this as a pledge to raise taxes on all Americans, not just on high earners and corporations as Mr. Biden has proposed.

The second quote is one from a Democratic debate in which Mr. Biden pledges a pathway to citizenship for 11 million undocumented immigrants. The Trump campaign says that such a policy would create competition for “American jobs.”

The takeaway: After tens of millions of dollars spent on a wide variety of attacks on Mr. Biden — from his relationship with China to his age — the Trump campaign is returning to its roots. Faced with polling deficits in key battleground states, the Trump campaign is reviving messages that were successful in 2016. Whether they connect with a pandemic-weary electorate remains to be seen.

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