WASHINGTON — First, it was Kamala Harris. Then, Joe Biden. Now, Pete Buttigieg.

All three are suddenly running ads on the air in Iowa, signaling both a new, quicker pace in the race for the 2020 Democratic presidential nomination and what Democratic insiders say is a need to check the formidable ground game of Massachusetts Sen. Elizabeth Warren.

"These campaigns have got to catch up with Elizabeth Warren in terms of organization," said Matt Paul, who ran eventual party nominee Hillary Clinton's successful Iowa caucus campaign in 2016.

While former Vice President Biden and Harris, a California senator, are at opposite ends of the diamond of four breakaway contenders for the first-in-the-nation caucuses and the party's nomination, they are both responding the way race participants might to keep pace, assess the strengths of the various competitors — including their own — and prevent any one of them from establishing too big a lead.

It's a dynamic that promises to play out as long as Biden, Harris, Warren and Sen. Bernie Sanders, I-Vt. — or any other candidates, like Buttigieg perhaps — remain competitive. And while Democratic strategists are less likely to talk about Sanders, he retains a solid base of enthusiasts in Iowa and around the country who can be activated, like Warren's, without the prompt of television or radio advertising.

Ed Slaughter, a former political pollster and professional cyclist who ran marathons on the side, said the spending on early ads — which are running more than four months before Iowans go to caucus — are one way for the candidates to measure themselves and one another.

"There will be surges during the miles of the race where they’ll be testing each other to see if the other runners in the marathon can keep pace — back and forth testing," Slaughter said. "I’m going to surge at a pace I know I can’t sustain for the rest of the race, so there’s a calculated risk."

Harris may be the easiest example of that so far. She has spent a little under $350,000 to date on ads in the state, almost all of it in the first week she went up on television. While she's still airing spots — avoiding the knock that she pulled down from TV — it would have been difficult for her to sustain her level of advertising through the caucuses in early February. That's true in part because commercial time becomes more expensive as there's more demand from candidates to buy it.

There's little question that Harris, who also ran a five-day bus tour in Iowa earlier this month, is in need of a boost both in the state and nationally right now. After a surge in interest following a strong debate performance at the end of June, she's seen her poll numbers slide. And if she can't make a solid showing in Iowa, Democratic operatives say, that could hurt her badly not only in the rest of the early states but also in the eyes of fellow Californians, who start early voting on the same day as the caucuses.

California has by far the most delegates available to the Democratic convention, and its voters will be watching the results in Iowa closely, according to Dave Jacobson, a Democratic strategist based in Los Angeles.

"The clock is ticking," he said. "She needs to have a strong performance in Iowa."

Biden's initial purchase, which trailed Harris' by a week or so, was for $500,000. Like Harris, he's not rolling in cash. She had a little over $13 million in the bank at the end of June, and he had a little bit less than $11 million.

Both campaigns should get a sense of how their messages are faring with Iowa voters — whether the ads work — pretty quickly. Harris' spot echoes Clinton's first Iowa ad in August 2015 in that it features her mother, and Clinton's 2007 "3 a.m." ad in that it discusses Harris' willingness to work at "3 a.m." so voters can rest easy. Biden's reminds voters of the themes he's emphasized since his rollout and at presidential debates: his ties to President Barack Obama and the idea that he is the most electable Democrat.

For Buttigieg, who had almost $23 million in the bank at the end of June, just running radio ads at the same time as Biden and Harris are up on television is a signal to caucusgoers that he is in the lead group, not back with the rest of the field. His polling in Iowa puts him in between those groups — at 7.5 percent in the Real Clear Politics average, he's 6 points behind Harris and 4 points ahead of Sen. Amy Klobuchar, D-Minn.

Biden is at 26 percent in that average, Warren is at 18 percent and Sanders is at 14 percent.

With Buttigieg's money outstripping his support, the broadcast play would appear to be a no-brainer. He's aiming his spots to coincide with his just-released plan for rural America, which is designed to tie into the Midwestern roots that distinguish him from the coastal candidates at the top of Iowa and national surveys.

While these three aren't the only candidates who have run ads in Iowa — several others have done so at various points to boost their name identification, or to show donors what they're doing with the money they've received — the cluster of high-profile contenders getting in at the same time represents a milestone in the 2020 race.

Both Harris and Biden have had trouble raising the kind of renewable small-dollar grassroots contributions that have fueled Warren and Sanders so far. Biden's initial buy of $500,000 is substantial enough to show he's serious but not enough to commit him to sinking his entire treasury into the state.

The challenge for all the candidates in Iowa, as it remains across the country, is that the race is unsettled enough to preclude picking up stakes and moving on to more inviting turf.

"It's pretty clear that Iowa is up for grabs and that no candidate has locked it down," one Harris aide said. "We want to be playing there, we want to compete to win there. I think you’re going to have to do pretty decently in Iowa to keep competing for the long haul."

CLARIFICATION (Aug. 21, 2019, 3:30 p.m. ET): In an earlier version of this story, one reference to South Bend Mayor, Indiana Mayor Pete Buttigieg's new broadcast ads seemed to suggest they are appearing on television. The story has been updated to clarify that they are radio ads.