After weeks of anxiety over Biden’s electoral chances,fired-up Democrats now believe they have a candidate who will be able to draw a contrast with Trump and make the election a referendum on the former president and his “MAGA Republicans”.

She’s a prosecutor,they will say;he’s a convicted felon. She’s pro-choice;he’s pro-life. She’s a champion for democracy;he has authoritarian tendencies. She’s 59;he’s 78,and now officially the oldest person in the running for president.

But the big challenge for Harris will be to define herself before others do,particularly as recent national polls show she trails Trump,and her approval rating has long been on a par with Biden’s.

What’s more,Harris has had presidential ambitions before,and entered the battle for the 2020 Democratic nomination with the same kind of hype and energy surrounding her now,only to fizzle out quickly.

After an initial “viral moment”,in which she called out Biden on the issue of race and segregation during a candidates’ debate in 2019,she struggled to articulate what she stood for,had a revolving door of disgruntled staff,and ended up dropping out before the first primary contest was held.

Her early days of the vice presidency were also rocky. Much to Harris’ chagrin,Biden saddled her with the task of stemming migration across the US-Mexico border from Central America.

But as illegal crossings surged,Harris stumbled,fumbled and occasionally lost her cool,most memorably during an interview with US TV newsman Lester Holt,who grilled her during her first overseas trip,in Guatemala,over why she had failed to visit the border as part of her work.

“I’ve never been to Europe,” she said,visibly fed up with his line of questioning. “I don’t understand the point you’re making.”

Just as Harris began laying out how she plans to take on Trump if,as expected,she wins the Democratic nomination next month,Republicans also provided a glimpse of how they plan to attack their new top rival.

First,they will continue to cast her as Biden’s “border tsar”,linking her to a crisis in which more than 3.2 million people illegally crossed into the US last year,far more than the 1.9 million when Biden first took office.

Second,they will seek to paint her as a “radical liberal” and someone who got her job due to Democrat identity politics rather than merit.

This was obvious on Monday (US time),when Tennessee congressman Tim Burchett described her as a “DEI[diversity,equity and inclusion] hire”,in reference to Biden appointing her after making an election promise to pick a woman as his running mate.

Third,they will attack her criminal justice record and her history of flip-flopping on policies such as parole and legalising marijuana.

And fourth,they will accuse her of a years-long cover-up to hide Biden’s fitness for office to American voters. As former Trump administration immigration adviser Stephen Miller put it in conspiratorial terms:“What did Kamala know and when did she know it?”

Despite the risks,Democrats have coalesced around the vice president and believe she will be an energetic campaigner who can rev up the base on issues such as abortion and democracy,while attracting centrist votes in key swing states,particularly if she picks a strong running mate,such as popular Pennsylvania Governor Josh Shapiro or astronaut-turned-Arizona senator Mark Kelly.

In a sign of the momentum building,Harris has now won support from enough delegates to secure the nomination,garnered an extra $81 million in donations,and has all but cleared the Democratic presidential field of any serious competition.

Former House speaker Nancy Pelosi has now also endorsed her,despite saying earlier that she would favour a competitive process to pick a presidential nominee.

And Biden,who is to return to work on Tuesday afternoon (US time) after isolating with COVID,called into his old campaign headquarters while she was there,to urge his former staff to get behind her.

“I’m watching you,kid,” he told Harris over the speakerphone. “I love you.”

Whether enough voters feel the same come November 5 is the big question.

Get a note directly from our foreign correspondents on what’s making headlines around the world.Sign up for the weekly What in the World newsletter here.