When U.S. Sen. Kamala Harris of California ended her presidential bid Tuesday, Julián Castro saw an opportunity.

Castro, the last Texan in the Democratic presidential race, appealed directly to supporters of Harris, who was once a top-tier candidate but then faded, saying they shared a common cause in championing those who typically are overlooked or taken for granted by the political system.

In a call with reporters Thursday, Castro said his campaign raised $360,000 from 18,000 people in the two days since Harris dropped out. He said Tuesday was his biggest fundraising day of the quarter.

The cash infusion means Castro has met the fundraising threshold to qualify for the next debate on Dec. 19, he said, but the former Obama administration Cabinet member still faces an uphill climb to meet the polling threshold. Castro has been stuck for months at about 1% in national polls.

“Although I’ve met the donor threshold, that is not enough,” Castro told reporters. “We’re asking folks to give their $10, $20, whatever they can to make sure that we have the resources necessary to boost our polling nationally and in these early states.”

Harris’ departure means that the debate stage for the first time this election could be all white, a development that could undermine the party’s efforts to project an image of diversity. No nonwhite candidate has qualified ahead of Thursday's deadline.

After Harris’ withdrawal, Castro and U.S. Sen. Cory Booker of New Jersey criticized the Democratic National Committee’s rules that have made it difficult for them to make it to the debate stage.

Booker, the remaining black candidate in the race, said he also saw an increase of financial support after Harris’ exit, saying Wednesday marked the best online fundraising day of his campaign.

Castro on Thursday asked for a complete overhaul of the nominating process in future elections, repeating his call to change the order of the contests. The first two nominating states, Iowa and New Hampshire, are overwhelmingly white.

“I actually don’t agree that we should change the rules in the middle of the game,” Castro said about changing the debate requirements. “We need to change the whole game.”

Appeal to Harris supporters

In interviews and fundraising emails, Castro amplified his message about the lack of diversity on the debate stage.

Castro praised Harris and called on her supporters to join his campaign.

“To all of Kamala’s supporters, I invite you to join us to continue the fight,” Castro wrote in a campaign email. “It was our shared values that brought us together in friendship, and I hope those same values inspire her supporters to find a home with this campaign.”

Gilberto Ocañas, a veteran of four Democratic presidential campaigns and a longtime Castro adviser, said he feels a renewed sense of optimism in calls with supporters.

It’s not clear if major Harris supporters or donors have moved to Castro’s campaign, but Ocañas said he thinks some are considering it.

“I do think there is some traction happening for (Castro) that hadn’t been there before,” Ocañas said.

A Castro campaign spokesman did not respond to a request for comment.

The road to Iowa

Now, Castro said, he’s focused on the Iowa caucuses on Feb. 3.

Robert Barron, a Castro supporter and special assistant to the president for government and community relations at Grand View University in Des Moines, said it’s too early to see how Harris supporters in Iowa will react and polling is “only somewhat informative.”

“What I would expect to see in Iowa is that a lot of caucusgoers are still making their decisions and that’s going to happen increasingly over the next two months.”

But, Barron said, Harris’ exit might give Castro’s campaign a chance to gain traction.

“I think voters are going to have to question the state of the field and whether they believe it’s fully representative,” he said. “This is why you stay in it at this point. These are the conditions that are right to make a big move.”