LOS ANGELES (Reuters) - Miles from where seven Democratic presidential hopefuls will take the debate stage on Thursday night in Los Angeles, Julian Castro on Wednesday walked the city’s Skid Row, the largest encampment of homeless people in the United States

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Castro, a former U.S. housing secretary, is among a handful of presidential candidates trying to find ways to stay in the conversation, after failing to meet the polling and donor requirements needed to qualify for Thursday night’s televised event.

U.S. Senator Cory Booker, among the 15 Democrats vying to win the right to take on Republican President Donald Trump in November 2020, is launching a new television ad on Thursday that references his omission.

“I won’t be on tonight’s debate stage, but that’s okay because I’m going to win this election anyway,” Booker says in the advertisement, which will air in several states including those holding the first four nominating contests - Iowa, New Hampshire, Nevada and South Carolina.

“This election isn’t about who can spend the most, or who slings the most mud. It’s about the people,” Booker says in the ad.

The Democratic Party has struggled with how to handle a historically large field of candidates. In total, more than 25 people have mounted campaigns, though the field has dwindled as some have bowed out.

The party has ratcheted up the requirements to participate in the debates, which mandate that they demonstrate mettle in fundraising and polling. For Thursday’s debate, candidates must show that they have received donations from at least 200,000 unique donors, as well as meeting polling benchmarks.

Twenty candidates qualified for the first debate back in June. By November that was down to 10. On Thursday, it will be down to seven.

The left-out Democrats have largely focused their ire on the rivals with the most personal wealth - specifically billionaires Tom Steyer, who will be participating in the debate, and Michael Bloomberg, a latecomer to the field who could not qualify for the televised debate because he is not taking any donations.

Bloomberg will campaign in Tennessee on Thursday, where he will unveil his policy proposal on health care.

Castro bemoaned the ability of billionaires who are self-funding their campaign to make the debate stage.

“Our campaign has been speaking up for people who are often left behind,” Castro said after his walk down some of the roughest blocks in Los Angeles. “What we are seeing in this election cycle is that, too often in politics, money still talks, that people can basically buy their way on to the debate stage.”