Former Vice President Joe Biden is leading the pack of 2020 Democratic presidential candidates, with Sens. Bernie Sanders and Elizabeth Warren tied behind him, according to a new NBC News|SurveyMonkey online poll.

The poll, conducted after the first Democratic debate in late June, shows 25 percent of respondents would vote for Biden if a primary or caucus was held today, while Sanders, I-Vt., and Warren of Massachusetts each garnered 16 percent.

Sen. Kamala Harris of California was close behind, with 14 percent, and South Bend, Ind., Mayor Peter Buttigieg registered at 8 percent. Former Texas Rep. Beto O'Rourke and Sen. Cory Booker of New Jersey each snared 3 percent, while the rest of the 24 person field came in at 2 percent or less.

Democratic 2020 presidential candidate Sen. Elizabeth Warren speaks to members of the media during a town hall at the Peterborough Town House in Peterborough, New Hampshire on July 8, 2019. Elizabeth Frantz / Reuters file

The poll was conducted between July 2 and July 16, after 20 of the Democratic candidates went head-to-head in the first primary debate in Miami, which was hosted by NBC News, MSNBC and Telemundo over two nights.

The survey also found that President Donald Trump's overall approval rating ticked upward to 48 percent in July, compared to 45 percent in a NBC News|SurveyMonkey online poll in September. The president's disapproval rating decreased to 51 percent, compared to 54 percent in the September poll.

The survey also shows the Trump administration's frequent messaging about a crisis on the border appears to be having an effect on public opinion.

A total of 22 percent of respondents said immigration is the issue that matters the most to them right now, up from 15 percent in September. That was slightly more than the number of people who identified health care or jobs as the most important issue — both garnered 21 percent. Fourteen percent said the environment was their top concern.

In September, 25 percent of respondents had identified jobs and the economy as the most important issue. The new poll found 68 percent of Americans said the condition of the national economy is "very good" or "fairly good," down 1 percentage point from September.

The poll found most Americans don't like Congress: Seventy-five percent of respondents "somewhat disapprove" or "strongly disapprove" of the way Congress is handling its job. That number was up a tick from 74 percent in September. Only 24 percent of respondents approved of the job the body is doing, about even with the September poll.

A majority of Americans are also in favor of keeping abortion legal. Asked if they would "like to see the Supreme Court completely overturn its Roe v. Wade decision," 64 percent of respondents said no.

A small majority were also in favor of paying higher taxes to improve schools and infrastructure in their states. Fifty-four percent said they'd be "very willing" or "somewhat willing" to pay higher taxes to improve public schools, while 59 percent said they'd be very or somewhat willing to pay higher taxes to fund improvements to roads and bridges.

The NBC News|SurveyMonkey national poll was conducted July 2-16 among 13,553 registered voters with an error estimate of plus or minus 1.2 percentage points. Among the 5,548 Democrat and Democratic-leaning registered voters, the margin of error is plus or minus 2 percentage points. Respondents for this survey were selected from the more than 2 million people who take surveys on the SurveyMonkey platform each day. For full result and methodology, click here.