Updated at 9:34 p.m. on Aug. 8: Revised to include an email to supporters from Joaquin Castro.

President Donald Trump attacked Democratic Rep. Joaquin Castro of San Antonio on Twitter on Wednesday afternoon following a tweet from Castro that listed the top donors to Trump's reelection campaign from San Antonio.

Trump, who was in El Paso on Wednesday visiting victims of Saturday's mass shooting, lit into Castro, calling him "the lesser brother of a failed presidential candidate."

Joaquin Castro has faced fierce criticism over a tweet that lists 44 people who contributed the maximum amount allowable under federal law to Trump's reelection campaign.

The tweet was sent Monday night from Castro's campaign account and includes the names of 44 San Antonio residents and the employers of many of the people, public information available on the Federal Election Commission site.

Other people were listed as "retired," "self-employed," "homemaker," "entrepreneur" or "information requested."

I don’t know who Joaquin Castro is other than the lesser brother of a failed presidential candidate (1%) who makes a fool of himself every time he opens his mouth. Joaquin is not the man that his brother is, but his brother, according to most, is not much. Keep fighting Joaquin! — Donald J. Trump (@realDonaldTrump) August 8, 2019

In the tweet, Castro said the donors were "fueling a campaign of hate that labels Hispanic immigrants as 'invaders.'"

Trump's tweet targeted both Castro brothers, writing "Joaquin is not the man that his brother is, but his brother, according to most, is not much."

Castro's twin brother, former Housing and Urban Development Secretary Julián Castro, is running to be the Democratic nominee for president. Joaquin Castro is his campaign manager.

Trump deleted his original version of the tweet that misspelled Castro's first name as "Juaquin."

Sad to see so many San Antonians as 2019 maximum donors to Donald Trump — the owner of ⁦@BillMillerBarBQ⁩, owner of the ⁦@HistoricPearl, realtor Phyllis Browning, etc⁩.



Their contributions are fueling a campaign of hate that labels Hispanic immigrants as ‘invaders.’ pic.twitter.com/YT85IBF19u — Joaquin Castro (@Castro4Congress) August 6, 2019

The tweet was posted two days after police say a gunman from Allen killed 22 people at an El Paso Walmart. Authorities have said they're investigating an anti-immigrant manifesto attributed to the shooter that said he was acting to stop the "Hispanic invasion of Texas."

Castro is already raising money over the Twitter attack by the president, sending an email to supporters Thursday evening that called Trump's tweet a "public temper tantrum" and asking for donations.

"Let's show President Trump that we're not going to stand for his divisive policies," the email stated. "I'm going to keep speaking truth to power."

Castro's campaign did not return multiple requests for comment.

Tim Murtaugh, director of communications for Trump's reelection campaign, responded on Twitter, calling on Castro to apologize for his tweet. Murtaugh also said Julián Castro should "disavow" the tweet.

"How low have Dems sunk?" Murtaugh tweeted Tuesday morning. "This is Joaquin Castro, Congressman & chair of his brother's campaign. Naming private citizens & their employers, targeting them for political views and exercising 1st Amendment rights."

How low have Dems sunk?



This is Joaquin Castro, Congressman & chair of his brother's campaign.



Naming private citizens & their employers, targeting them for political views and exercising 1st Amendment rights.



Should delete & apologize. Castro campaign should disavow. pic.twitter.com/5MSJPBBaF0 — Tim Murtaugh (@TimMurtaugh) August 6, 2019

Castro stood by his original message in a reply to Murtaugh several hours later, pointing out his tweet didn't share any private information.

"What I said is true — your campaign has stoked fear of brown-skinned immigrants," Castro tweeted. "Those contributions [have] been used to pay for over 2K @Facebook ads declaring an invasion by Hispanics. That is truly dangerous for millions."

Some prominent Texas Republicans have taken shots at Castro on Twitter. Texas Senators Ted Cruz and John Cornyn both derided Castro over his tweet. Gov. Greg Abbott tweeted it was a "perfect night" for Bill Miller Bar-B-Q, whose owner was among the list of donors.

U.S. Rep. Chip Roy, R-Austin, also responded to Castro's tweet, telling San Antonio residents: "I got your back."

"I will fight for freedom for you, including making sure you are not doxxed by public officials," said Roy, whose district runs the length of Interstate 35 from northern San Antonio to Austin. "Let's re-claim America from the clutches of hopelessly political leftists who prefer sanctuary & tent cities to strong communities."

Hey San Antonians... I got your back. I will fight for freedom for you, including making sure you are not doxxed by public officials. Let’s re-claim America from the clutches of hopelessly political leftists who prefer sanctuary & tent cities to strong communities. #Tx21 https://t.co/me0uFKL1D0 — Chip Roy (@chiproytx) August 6, 2019

Many of the replies from the public to Castro's tweet were similar to those of Murtaugh and Roy. Users accused Castro of "doxxing" those on the list. The term refers to publishing private information about people, usually with malicious intent.

Some users voiced support for Castro and stated their intent to boycott the businesses on the list, but many of the replies were negative.

An individual can donate up to $2,800 to a candidate per election. Individuals can donate an additional $5,000 to a political action committee supporting a candidate, with no limit on the number of PACs they can donate that amount to.