Shortly after New Jersey Sen. Cory Booker said President Trump is "responsible" for Saturday's deadly mass shooting in El Paso, Texas, another Democratic presidential contender, Julian Castro said "there's one person that's responsible directly" for the massacre -- "and that's the shooter."

At the same time, Castro told ABC News' "This Week," Trump has embraced "division and bigotry and fanning the flames of hate" as a form of "political strategy."

"It's no accident that, just a few weeks after he announced his 2020 reelection bid, there he was indulging and entertaining this 'Send her back' chant," Castro said. "And he's spoken about immigrants as being invaders. "He's given license for this toxic brew of white supremacy to fester more and more in this country. And we're seeing the results of that."

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In January, Trump wrote on Twitter, "Humanitarian Crisis at our Southern Border. I just got back and it is a far worse situation than almost anyone would understand, an invasion!"

And at the White House in March, Trump remarked, “Last month, more than 76,000 illegal migrants arrived at our border. We’re on track for a million illegal aliens to rush our borders. People hate the word ‘invasion,' but that’s what it is. It’s an invasion of drugs and criminals and people. You have no idea who they are.”

Acting White House chief of staff Mick Mulvaney denied earlier on "This Week" that Trump had "downplayed" the threat of white nationalism.

After the New Zealand mosque shootings in March, Trump was asked whether white nationalism was "rising threat around the world." The president responded: "I don’t. I don’t really. It’s a small group of people...But it is a terrible thing.”

Castro, speaking to anchor Jonathan Karl, said that only the shooter bears "direct" responsibility. (In a statement released later Sunday, Castro echoed that comment, saying, “These shooters are ultimately to blame for their actions. They are attempting to terrorize us but I believe that the vast majority of Americans reject this hatred.")

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"At the same time, as our national leader, you have a role to play in either fanning the flames of division or trying to bring Americans of different backgrounds together," Castro told Karl. "Most presidents have chosen to try and bring people together. This president very early on made a clear choice to divide people for his own political benefit. And these are some of the consequences that we're seeing of that."

Asked about the March interaction, Mulvaney said Trump has been misinterpreted.

"I don't believe that's downplaying it, look at what he said," Mulvaney said. "Look, this is not the same as international nuclear weapons. This is a serious problem, there's no question about it. But they are sick, sick people and the president knows that."

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Mulvaney continued: "I don't think it's fair to try and lay this at the feet of the president. There are people in this country this morning thinking that President Trump was happy by this. That's a sad, sad state of this nation. He's angry. He's upset. He wants it to stop. I don't think it's at all fair to sit here and say that he doesn't think that white nationalism is bad for the nation. These are sick people. You cannot be a white supremacist and be normal in the head."

Responding directly to Mulvaney's comments, Castro told Karl, "You know, it's so unfortunate that not only our president, but his administration can't rise up to the challenge of leadership in these times."

Separately on Sunday, Democratic presidential contender Pete Buttigieg pointed specifically to “weak gun safety” measures and white nationalism as the culprits, after the El Paso shooter was linked to anti-Mexican statements.

“This is terrorism and we have to name it as such,” Buttigieg said, specifically calling it “white nationalist terrorism” in a conversation with host Chris Wallace on “Fox News Sunday.”

“We need to acknowledge that this is a problem.” Buttigieg said, claiming that white nationalism has been “condoned at the highest levels” in Washington.

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“Right now you see it being echoed by the White House and there is a measure of responsibility that you just can’t get away from,” he said. Buttigieg cited President Trump’s comment that there were “very fine people” on both sides after a deadly attack at a white supremacist rally in Charlottesville, Va.

Trump condemned the El Paso shooting early Sunday morning, calling it "hateful" and "an act of cowardice."

"God bless the people of El Paso Texas," Trump said. "God bless the people of Dayton, Ohio."

Fox News' Ronn Blitzer contributed to this report.