Washington (CNN) Democratic presidential candidate and El Paso, Texas, native Beto O'Rourke said that President Donald Trump was a white nationalist after a mass shooting in El Paso on Saturday left 20 dead and a separate rampage hours later killed nine more in Dayton, Ohio.

CNN's Jake Tapper on Sunday asked O'Rourke, a former El Paso congressman: "Do you think President Trump is a white nationalist?"

"Yes. I do," O'Rourke said on CNN's "State of the Union." The Democrat also referenced Trump's record of insulting Mexicans as "rapists" and describing asylum seeking migrants as an infestation.

"The things that he has said both as a candidate and then as the President of the United States, this cannot be open for debate," he said.

Early Sunday morning, a shooter opened fire in Dayton, Ohio, killing nine people in the city's Oregon District, a popular downtown area. The shooter, a 24-year-old man, was shot and killed by responding officers. Hours before on Saturday, a shooter opened fire at an El Paso, Texas, shopping center, killing at least 20 people. A 21-year-old white supremacist is in custody in the Texas domestic terrorism case.

Trump called the El Paso shooting an "act of cowardice" on Saturday and said there "are no reasons or excuses that will ever justify killing people." On Sunday, the President tweeted: " God bless the people of El Paso Texas. God bless the people of Dayton, Ohio ."

O'Rourke on Sunday also referenced FBI Director Chris Wray's testimony that the number of hate crimes in the United States has increased.

"We have a problem with white nationalist terrorism in the United States of America today," O'Rourke said, adding that "these are white men motivated by the kind of fear that this President traffics in."

Referencing Trump's comments that " very fine people " marched alongside the white supremacists and neo-Nazis who rallied in Charlottesville in 2017, O'Rourke accused Trump of "sending a very public signal to the rest of this country about what is permissible, and in fact, even what he encourages to happen."

O'Rourke argued that it would take "all of us -- Republicans, Democrats, independents alike -- rising up, standing up to be counted against what this President is doing, against this white nationalist racism, against this violence and getting this country back."

He called for a shared "responsibility to call that out, to make sure that the American people understand what is being done in their name by the person who holds the highest position of public trust in this land."

"(Trump) does not even pretend to respect our differences or to understand that we are all created equal," O'Rourke told Tapper. "He is saying that some people are inherently defective or dangerous."

White House adviser Kellyanne Conway called for unity on Sunday, tweeting , "We need to come together, America."

"Finger-pointing, name-calling & screaming with your keyboards is easy, yet... It solves not a single problem, saves not a single life," she added. "Working as one to understand depraved evil & to eradicate hate is everyone's duty. Unity. Let's do this."

2020 Democrats respond

Later Sunday, Tapper posed the question of whether they thought Trump was a white nationalist to other 2020 Democratic presidential candidates.

South Bend, Indiana, Mayor Pete Buttigieg told Tapper that "at best (Trump is) condoning and encouraging white nationalists."

The mayor said Trump "made his career, politically, on demonizing Mexicans and now we're seeing reports that the shooter yesterday had his goal as killing as many Mexicans as possible."

"It is very clear that this kind of hate is being legitimized from on high," Buttigieg added. "And if that were not true, the President would be acting and speaking very, very differently than what he's doing right now."

New Jersey Sen. Cory Booker told Tapper "because this was a white supremacist manifesto," he wanted "to say with more moral clarity that Donald Trump is responsible for this."

Trump, Booker said, "is responsible because he is stoking fears and hatred and bigotry," is "failing to condemn white supremacy and see it as it is," and "has failed to do anything significant to stop the mass availability of weapons to people who intend to do harm."

When asked whether he thought there was a link between the President and the weekend's violence, Texas Democrat Julián Castro, who previously served as mayor of San Antonio, said, "I believe that President Trump is making it worse."

"Look, the person that is responsible for the shooting is the shooter," Castro said. "At the same time, if you're in a position of leadership, you set the tone for the country and there is no question that this President is setting a tone of division and fanning the flames of bigotry and hate."

He also slammed Trump as sowing division for political gains, saying, "Sometimes, for some people, and I believe this goes for the President, division is a political strategy. Bigotry is a way of stirring some people up so that they'll vote for you -- that's dangerous."

Vermont Sen. Bernie Sanders said he does think Trump is a white nationalist, saying, "All of the evidence out there suggests that we have a President who is a racist, who is a xenophobe, who appeals, and is trying to appeal, to white nationalism."

"It breaks my heart to have to say that this is the person we have who is President of the United States," he said.

And Ohio Rep. Tim Ryan told Tapper: "Well, the white nationalists think he's a white nationalists. And that's the crux of the problem. They support him."

California Sen. Kamala Harris stopped short of calling Trump a white nationalist, but stressed that the President's rhetoric has negative consequences.

She accused Trump of using the "microphone in a way that is about sowing hate and division in our country, in a way that is about not acknowledging domestic terrorism when it occurs."

Trump "does not understand the responsibility which comes with the office, which is to be a leader on every level -- including encouraging, challenging us to be our best selves," she told Tapper on Sunday.

In a tweet, former Vice President Joe Biden did not mention Trump, but called for unity in a fight to regulate assault weapons and against the National Rifle Association's influence.

"Assault weapons and high-capacity magazines don't belong on our streets," Biden wrote Sunday. "We must stand up to the @NRA and gun manufacturers—they don't own this country. We the people own this country."