Lots of voters decided to "swipe right" on Kamala Harris and Julián Castro after the first Democratic debates — signaling in polling and donations that they're interested to learn more about these candidates.

Why it matters: Now the pressure is on to make a lasting impression and keep up this post-debate momentum as much as they can before the next debates on July 30 & 31.

That's especially true for Harris, who's eating away at Joe Biden's support, and for someone like Castro who was basically unknown to the masses just last week.

Harris' strategy, per her campaign, is to keep her in front of new voters and spend money. "We are foot on the gas," texted Ian Sams, national press secretary for her campaign.

She just finished another Iowa tour and a few stints in Houston, New Orleans, and South Carolina — all in one week.

They're also investing heavily in Iowa, which is counter to her early strategy of spending more time in places like California and Texas.

They now have 65 staffers on the ground in Iowa "to harness energy into organizing capacity coming off the debate," Sams said, adding that they believe it might be the largest Iowa team of any 2020 candidate right now.

Castro's campaign says he's not changing a thing. "Secretary Castro isn't changing the way he campaigns ... [just] because he has a bigger spotlight," said Sawyer Hackett, the campaign's national press secretary.

The aggressiveness of his debate-night battle with Beto O'Rourke may have surprised voters and the media, but Castro maintains that's who he really is — especially passionate when it comes to immigration .

Castro plans to unveil at least two new policy proposals before the end of the month, including on climate change and gun violence prevention, his team said. He's also adding stops in Florida and Texas.

And now the campaign has press embeds traveling with them everywhere they go — a stark departure from just a few weeks ago when they'd have to beg for that kind of media attention.

The momentum for these two candidates was clear immediately after the debates.

Every post-debate poll has Harris firmly in third place, polling as high as 20% in some and within striking distance of Biden in others. Just days before the debates, she was polling in single digits.

FiveThirtyEight found roughly 25% of Harris' "new" supporters — those who said she's their first choice after the debate — previously said they were on Team Biden.

In one HuffPost/YouGov poll of 1,000 people, 29% of voters said their opinion of Castro improved after the debate. That includes 24% of independents and 11% of Republicans.

The buzz is seen in events, too. Castro's campaign planned "an intimate meet-and-greet" for 100 people at a mall in Texas this week, and 350 people showed up.

The bottom line: Presidential debates this early in the cycle matter because they drive media coverage, which leads to shifts in polling and surges in fundraising. But the next debates could reward someone else entirely.

Go deeper: What you need to know about the 2020 presidential candidates, in under 500 words