/ Updated 
By Suzanne Gamboa

WASHINGTON -- Housing Secretary Julián Castro, considered a potential Democratic vice presidential candidate, criticized GOP presidential hopeful Donald Trump for "demonizing immigrants," and took a swipe at the Republican party while he was at it.

Castro, former mayor of San Antonio, compared Trump's political methods to other Republicans, including former Arizona Gov. Jan Brewer, former California Gov. Pete Wilson, Rep. Steve King, R-Iowa.

He dismissed Trump's visit to the border city of Laredo, Texas, Thursday as unlikely to be a productive use of anyone's time.

"He's just trying to gin up political support," Castro told NBC News.

At the border, Trump projected Hillary Clinton would be the Democrats' presidential nominee and said he best way for Republicans to win the White House in 2016 was for him to win the GOP nomination and Clinton.

He also said that he had been warned that his trip to the border was dangerous and questioned about making the trip. "I have to do it. I have to do it," Trump said.

Meanwhile, U.S. Rep. Joaquín Castro, D-Texas, the housing secretary's identical twin, used Twitter to take on Laredo Mayor Pete Saenz, who appeared with Trump at his news conference in the city.

Earlier in the day, Saenz had discussed his city's safety and his plans to "receive" Trump.

"We're a safe city. The border in general, it's a misconception. It's also a misconception that there are gun battles here because of the cartels," Saenz told MSNBC's Jose Diaz-Balart.

During the news conference, however, local officials said Laredo was getting national attention because of Trump. "We're excited to have Mr. Donald Trump here in Laredo, Texas. It's provided us an opportunity to present to Mr. Trump and his entourage what Laredo is all about.

"We appreciate all the attention, but we know the attention comes primarily through this gentleman here," Saenz said.

Laredo's City Manager Jesus Olivares also spoke at the news conference alongside Trump. Asked whether he agrees with Trump on buildnig a border wall, Olivares said "we don't think that's necessary at this time."

Another Laredo native, Roger Rocha Jr., the newly elected president of League of Latin American Citizens, said it's easy for someone to posture while running for office, but that Trump was preying on the fears of people in the U.S. LULAC organized protests against Trump.

"He took five deferments (not) to go to Vietnam because he didn't want to walk in the jungle as other brave men and women did. So he wants to walk around the riverbanks of Laredo. He's trying to make up for those five deferments by walking the banks of the Rio Grande. iI's not going to work. People see right through him," Rocha told NBC News.