That was much better than Castro, 44, the only Latino in the 2020 race, has fared in other polls on the Democratic primary race that polled fewer Latinos.
Leading the poll was former Vice President Joe Biden (59 percent), followed by Sen. Bernie Sanders, independent from Vermont (58 percent) and former Texas Rep. Beto O’Rourke (48 percent). The three have led other polls of the crowded 2020 field.
The poll released Tuesday was "specifically designed to have a large and accurate sample of the Latino community, offered in English and Spanish, and weighted to match the correct census demographics," said Matt Barretto, founder and principle of Latino Decisions, which has done work for Democrats. "No other poll in 2019 which has reported a Latino sub-sample has done that."
Latino Decisions surveyed 606 Latino registered voters nationwide from April 9-15. The poll has a margin of error of plus or minus 3.9 percent.
“What we learned in Nevada in 2010 was that when the so-called mainstream polls have small or inaccurate Latino samples, it skews the overall results,” said Latino Decisions pollster Matt Barreto.
While it is early and voters are still getting to know candidates, how the candidates fare in national polls is critical as they vie for spots in Democratic party debates, the first of which will be held in June and hosted by NBC News, MSNBC and Telemundo.
The party has said candidates can qualify for the first two primary debates if they register at least 1 percent in three national polls, which Castro's campaign said he has done.
But the number of Latino or African American voters included in many national polls often are small leading to higher margins of error for those groups. That raises questions about whether candidates of color are at a disadvantage if they have heavy support in minority communities.
“What we learned in Nevada in 2010 was that when the so-called mainstream polls have small or inaccurate Latino samples, it skews the overall results,” Barreto said.
“In Nevada, we noticed other polls were missing critical support numbers for Harry Reid and they wrongly predicted he would lose in 2010," he said, referring to the former senator.
The Morning Rundown
Get a head start on the morning's top stories.
The Democratic Party has provided an alternative for debate qualification: at least 65,000 unique contributors and a minimum of 200 unique donors per state in at least 20 states. Puerto Rico will count as a state. Candidates can qualify with either threshold, but if more than 20 candidates qualify, they will have to meet the fundraising threshold, too.
Xochitl Hinojosa, the Democratic National Committee's communications director, said the DNC's goal has been to be as inclusive as possible for the first two debates, which is why the party added the fundraising threshold. That threshold is "another indicator of support for a candidate that's not always captured in polling so early in a cycle," Hinojosa said.
"Historically, both parties have relied just on polling for debates, but we believe it's also important for a nominee to have a strong grassroots fundraising infrastructure in order to beat Donald Trump," she said.
The number of potential Latino voters is growing as some 1 million young Latinos turn 18 every year and as the political climate has helped mobilize them. The 2018 midterm elections also saw a spike in Latino voter registration that influenced several elections.
In addition, Latino voters will have an early impact on the 2020 primary. Iowa and New Hampshire will hold the first caucus and primary, followed by Nevada, a state with heavy Latino population, on Feb. 22. California and Texas — the two states with the highest number of registered Latino voters in the country — are on March 3, Super Tuesday.
“If your polling lacks an accurate and robust Latino sample, your overall picture of the Democratic electorate is skewed,” Barreto said.
Maya Rupert, Castro’s campaign manager, said Castro’s showing in the Latino Decisions poll “is exactly the momentum we’re describing we’re seeing.”
The Castro campaign reported raising $1.1 million in the first quarter of January through March 31, lagging far behind other candidates, including candidates with far less name recognition.
But the campaign has said it raised another $572,000 in the first two weeks of April.
“As Secretary Castro’s name ID grows, his favorability grows. That is exactly what we are seeing in this poll,” Rupert said. “With the role the Latinx community is going to play this cycle, it really is significant who folks in that community are excited about.”
Bernard Fraga, an associate professor of political science at Indiana University, Bloomington, said it is way too early to be looking at polls as a predictive of who the Democratic nominee might be. A candidate having less than 1 percent or 2 percent is not statistically significant, he said.
Favorability and name recognition are more important metrics because they show the variation among voters, said Fraga, author of “The Turnout Gap: Race, Ethnicity and Political Inequality in a Diversifying America.”
“Voters want to support candidates they think can win and defeat Trump, according to polling, but the decision between candidates is much more fluid than polls would indicate,” Fraga said.
Castro registered the least “unfavorable” ratings among all the candidates with just 18 percent. That gives him the best favorability ratio — the higher the ratio, the more you are liked than disliked.
“When we measure his favorables against his unfavorables, he registers a ratio of 2.5 to 1, higher than Biden, 2.1 to 1, and Sanders, 2.07 to 1, and O’Rourke, 2 to 1,” Barreto said. “This suggests there is an upside for Castro if he can increase his name recognition to a larger national Latino audience.”
In his home state, Texas, Castro fares even better. His favorable rating is 59 percent, to 14 percent unfavorable, for a ratio of 4.2 to 1. But because he trails in fundraising, "he is going to work harder with less resources," Barreto said.
Castro ranked below five other candidates in name recognition, according to the survey.
Sixteen percent of voters said they had not heard of Castro, compared to Biden, who 2 percent had not heard of; Sanders, 5 percent; and Sen. Elizabeth Warren, D-Mass., 11 percent. Twelve percent of Latino voters said they had not heard of O’Rourke.
Rupert said she expects Castro to get a “huge amount” of support in communities of color. His campaign has gone out of its way to talk about issues that affect communities of color — some that affect people across the board, Rupert said, citing Castro’s trip to Puerto Rico and focusing his first policy rollout on immigration and police violence and its impact on communities of color.
“So I think you see in this cycle there are a lot of issues relevant to communities of color he’s been leading on and you will see a huge amount of support," she said. "I do think some of these polls do a disservice by not allowing us to see that."
Suzanne Gamboa is a national reporter for NBC Latino and NBCNews.com
Stephen Nuño-Pérez is an NBC News contributor and political scientist. Stephen is an Associate Professor and Chair in the Department of Politics and International Affairs at Northern Arizona University.