Aamer Madhani | USA TODAY

USA TODAY

With Rep. Eric Swalwell announcing Monday that he will be the first 2020 Democratic presidential contender to drop out, the race for the White House is entering a new phase: the culling of the herd.

Swalwell made the announcement at his campaign headquarters in Dublin, California, and said he would instead focus on getting reelected to the U.S. House and helping other Democrats win election.

The three-term congressman's decision came as questions about the health of his campaign swirled after he abruptly canceled campaign events last week in the early-voting state of New Hampshire.

"We have to be honest about our own candidacy's viability," Swalwell said. "Today ends our presidential campaign, but it is the beginning of an opportunity in Congress with a new perspective shaped by the lives that have touched mine and the campaign throughout these last three months to bring that promise of America to all Americans."

Beyond Swalwell's campaign, this moment marks an inflection point in the 2020 campaign in which many candidates at the back of the crowded field are beginning to reckon with crystallizing truths: The window to emerge as a serious contender is closing and the winnowing of the field is inevitable.

"Some may be hanging on to the hope that if they can just get past the summer doldrums, they can catch voters when they start tuning in come fall," said Susan MacManus, a political scientist at the University of South Florida. "But after that, it becomes much, much more difficult especially for some of these candidates who will be facing pressure from party leaders to drop out and get into (another race)."

An interactive guide: Who is running for president in 2020?

For many candidates whose campaigns have been lingering in the low single digits or worse, the time is quickly approaching where they must make an honest assessment of not only whether they can run a credible campaign, but also how they're going to approach the next step in their political career, MacManus added.

Polls in the aftermath of the first round of debates saw the front-runner and former Vice President Joe Biden's lead shrink, Sen. Kamala Harris surge and Sen. Elizabeth Warren solidify her spot as a top candidate.

Top-tier candidates Sen. Bernie Sanders and South Bend, Indiana, Mayor Pete Buttigieg have watched their poll numbers slip in recent weeks, but they remain among the strongest fundraisers. Meanwhile, Sen. Cory Booker, Sen. Amy Klobuchar, and Former Rep. Beto O'Rourke continue to register in the low single digits in polls.

And then there is the rest of the field.

None of the more than a dozen bottom-tier candidates managed to deliver a momentum-building moment needed to move beyond also-ran status.

"We need a candidate that is tested, I'll tell you that much," Swalwell said. "Donald Trump is the best political puncher ever in American politics. Whoever we send to that debate stage with President Trump is going to have to be able to take a punch, throw a punch and then unite the country at the end of the campaign."

Making the debate stage

While Swalwell managed to make the stage for the first round of debates in Miami late last month, he was far from a lock for the next round, which will take place in Detroit July 30 and 31.

The crowded contest to just get onto the Detroit debate stage could force the Democratic National Committee to employ a tiebreaker with more than 20 candidates set to meet the threshold.

The DNC has capped the debates at 20 participants over two nights and required candidates earn at least 1% in three qualifying polls or contributions from at least 65,000 donors.

Three major Democratic party candidates were left out in the first round of debates — Rep. Seth Moulton of Massachusetts, Montana Gov. Steve Bullock and Miramar, Florida, Mayor Wayne Messam — because they did not meet either threshold.

DNC officials say they would look to “the total number of polls in which each candidate received 1 percent or more support” as a tiebreaker. Swalwell has broken the 1% margin in three polls, fewer times than several other candidates near the bottom of the polls, including New York City Mayor Bill de Blasio, Colorado Sen. Michael Bennet and Bullock.

It only gets harder for candidates as summer turns into fall.

For debates scheduled for September and October, candidates will have to hit 2% in four qualifying polls and tally at least 130,000 individual donors, according to rules set by the DNC.

Swalwell said that he was not confident that he was going to meet the higher donor threshold to compete in the fall debates. He said he raised a paltry $850,000 in the second quarter from about 21,000 individual donors.

"We wanted to be honest with ourselves and our supporters," he said. "If there was a viable chance, I would not be standing here today. From day one, I was running to win."

Swalwell campaign high point

Swalwell, 38, who was seen as a longshot candidate since entering the crowded field in April, had his biggest moment in the campaign less than two weeks ago, when he took on Biden during the first debate.

"I was 6 years old when a presidential candidate came to the California Democratic Convention and said, 'it’s time to pass the torch to a new generation of Americans.’ That candidate was then senator Joe Biden," Swalwell said. "Joe Biden was right when he said wit was time to pass the torch to a new generation of Americans 32 years ago. He’s still right today.”

Saul Loeb, AFP/Getty Images

Biden pushed back, saying he still has ideas to move the country forward. The line captured national headlines for Swalwell, who has hovered at 1% or less in national and early state polls, but it did not spur any momentum for his campaign.

Swalwell was one of three candidates – de Blasio and Ohio Rep. Tim Ryan were the others – who failed to attract a single supporter in a Suffolk University/USA TODAY survey of voters in the first-in-the-nation caucus of Iowa conducted following the June 28-29 debates.

He also failed to register support in post-debate national polls conducted by CNN and Quinnipiac University.

Post debate shakeup: Poll: Kamala Harris surges in Iowa as Bernie Sanders suffers after debate

The congressman is hardly alone in his struggle to break through with voters.

Former Colorado Gov. John Hickenlooper on Sunday acknowledged during a town hall in Perry, Iowa, that some of his staff have urged him to give up on the White House and run for the U.S. Senate against Sen. Cory Gardner, one of the most vulnerable GOP senators up for reelection in 2020.

“I realize I’m at 1% in the polls, and you know some of my own staff said, ‘Run for Senate’! I think Iowa is where that can be changed,” he said.

Hickenlooper replied to one voter at the town hall that “you’ve gotta love it to give your life to public service,” and suggested he’d prefer the work of being president to that of being a senator. Several high-ranking aides quit his campaign last week, and Hickenlooper said Sunday that the "vast majority of the problem with the campaign was me not being as good of a messenger as I need to be."

Wilfredo Lee, AP

MacManus, the University of South Florida political scientist, said that Hickenlooper has left himself enough space to be nudged by party leadership into the Senate race.

"I think the way he's handled this has been really smart," MacManus said.

Swalwell announced his candidacy in April with an appearance on the "The Late Show with Stephen Colbert," and has poured much of his time and attention into Iowa, his birth state and a place he argued that he could connect with voters. He made 20 visits to the state between 2016 and his announcement.

On the stump, Swalwell has been outspoken critic of President Trump, while also seeking to make the case that it was time for voters to give a candidate of a younger generation the opportunity to try to fix problems like gun violence, climate change and rising higher education costs.

The father of two young children, Swalwell often brings up that he he still owes more than $100,000 in student loans as he made the case that he personally understands the struggles of many working Americans.

He had proposed a pathway for students to attend college debt-free if they participate in work-study programs and commit to community service.

Swalwell also invoked the younger generation of activists that emerged following last year's mass shooting at Parkland, Florida, as inspiration for tightening gun laws if he were elected president.

Biden took to Twitter Monday after Swalwell announced he's bowing out to praise the congressman for elevating the issue of combating gun violence and his work pushing for tightening election security from his perch in Congress.