Ms. Harris, who is now officially the Democratic Party's nominee, announced Minnesota Governor Tim Walz as her candidate for vice president.

Let's hear what she had to say.

I stand before you today to proudly announce I am now officially the Democratic nominee for president of the United States.

Our campaign is not just a fight against Donald Trump.

Our campaign, this campaign, is a fight for the future.

Well, prior to this moment, Tim Walz was not a household name in the U.S. and around the globe.

He served in the Army National Guard for 24 years, reaching the position of command sergeant major, one of the highest enlisted ranks in the military, and was a former social studies teacher and football coach before becoming a U.S. congressman and serving for 12 years.

He was then elected governor of Minnesota in 2018.

Joining Ms. Harris on stage in Philadelphia, he gave an impassioned speech.

She believes in the opportunity for every single person to join the middle class.

She believes in the promise of America.

We just got to fight.

We just got to fight.

Because as soon to be President Harris says, when we fight, we win.

Well, you get a sense of the excitement at that event in Philadelphia and from Democrats across the country.

Rohini Kosoglu is a former close advisor to Vice President Kamala Harris.

She told the BBC that she's used her own experience as vice president to inform her pick of running mate.

One thing that's so important to her throughout her time is that she's a vice president for all people.

It doesn't matter where you live.

She's the vice president, and she's going to do her best to make sure that people know that she's fighting for them.

And I think she'll want to make sure that Governor Walz does the same in this role and that he goes to meet people where they are.

He makes sure that nothing is taken for granted.

It was important that one of the concepts she talked to him about was about that they're an underdog team.

And it's really important for Americans to know that their vote is not taken for granted and that they're going to show up and earn their right to vote.

Well, let's get more now from our North America correspondent, David Willis.

Hello to you, David.

So we now know the team, as it were, and they've got a very busy agenda ahead.

Talk us through the reaction so far.

Well, you're absolutely right.

They do have a very busy agenda ahead, Sally.

That rally in Philadelphia marking the start of a marathon five-day sprint in which the two of them will be taking in seven of the key battleground states.

Kamala Harris herself made the point in her remarks today that this has become a whirlwind of a campaign.

And it's a campaign which, of course, got underway just over two weeks ago.

But she made the point today that it was a fight for the future, as she put it, with issues such as greater gun control laws in this country, affordable health care for all, reproductive rights and so on, all at issue in this campaign.

And she said she had been searching for a partner, somebody who could build with her a brighter future for the United States of America.

And she said she had found such a partner in Tim Walz.

But he has some way to go, as you mentioned in your introduction there, Sally, to introduce himself to the American people.

There was a poll conducted by National Public Radio which showed that 70 percent of the American public either have no opinion about Tim Walz or have no idea who he is.

OK, interesting.

Thank you, David.

Well, let's get the view now of Democratic strategist Amanda Renteria, who was political director for Hillary Clinton's presidential campaign.

And if we just pick up on David's point there that nobody knows much about Tim Walz, is that a good thing?

It's great because it really gives the country a chance to get to know who Tim Walz is.

And given the sprint of this election, the kind of coverage Kamala has been getting from the very beginning and everybody waiting for this announcement, it's really an opportunity for people to really see who Tim Walz is, not only today, but over the next five days where they will be in all the battleground states.

And frankly, when you meet him, you just like the guy.

And I think what's going to happen over the next couple of days is you are going to see a lot more about his experience, his policies, how he's governed the state of Minnesota.

And so it's quite an exciting time for the Democratic Party.

The Trump campaign is already on it, describing him as dangerously liberal extremist.

Will that gain any traction?

It's quite hard to gain that kind of traction with a guy who is a hunter, who has been in the military for 24 years, who really has had to govern a state where he's had to work with both sides of the aisle.

So I don't think that's going to stick.

And I think people are tired of hearing the same attack from this Trump team.

And I'm quite surprised it's still the same attack.

I mean, he does tick every box.

He's been a schoolteacher, a football coach.

He's a dad.

He's been in the military.

As you say, he's a hunter.

I mean, he kind of, you know, you can understand why Kamala Harris is pretty keen on him.

But at the end of the day, when it comes to what people do in November, it is all about Donald Trump and Kamala Harris, isn't it?

The VPs don't make that much of a difference.

You know, I said this in this election from the very beginning.

This election might be quite different.

When Biden was in it, it mattered when you're working with 78-year-olds or 80-year-olds who are representing the country.

It does matter, actually, who the vice president is.

So for Trump, I think the vice president has mattered.

It has affected his campaign already.

For Kamala, you're right.

I think we are seeing a ticket where people are going to mostly focus on Kamala.

But the vice president actually does matter on the Trump side of the ticket.

Just quickly, what do you know about J.D.

Vance, who is the VP running with Donald Trump?

I mean, when we see Tim Walz versus J.D.

Vance, for example, in a TV debate, how's it going to look?

I think what you're going to see is a wise, joyful military veteran who has fought for kids, a high school teacher and coach, very much comes to mind.

And when you see J.D.

Vance, you really do see a very young, angry, frustrated, someone who looks like Trump and uses the same kind of tactics.

And so I think that's going to come out pretty clear.

I think the most important thing, though, especially 90 days into an election, is the joy that is coming from Tim Walz, from the Democratic Party, from Kamala Harris.

That is what really gets the energy going when you need it right now as we start to vote.

OK, Amanda, thank you for your time and your perspective.

Just to remind you that Amanda Renteria was political director for Hillary Clinton's presidential campaign.