Former vice president Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE leads Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE in the latest Hill-HarrisX poll released Monday by 19 points, maintaining a clear separation from the rest of the field in the Democratic presidential primary,

Biden won 35 percent support from Democratic and Democratic-leaning independent registered voters in the June 1-2 survey, compared to 16 percent for Sanders (I-Vt.).

South Bend, Ind. Mayor Pete Buttigieg Pete ButtigiegThe Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice Hillicon Valley: FBI, DHS warn that foreign hackers will likely spread disinformation around election results | Social media platforms put muscle into National Voter Registration Day | Trump to meet with Republican state officials on tech liability shield MORE was a distant third with 8 percent. He was followed by Sen. Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (D-Mass) at 5 percent.

Sen. Kamala Harris Kamala HarrisHarris faces pivotal moment with Supreme Court battle Nearly 40 Democratic senators call for climate change questions in debates Joe Biden has long forgotten North Carolina: Today's visit is too late MORE (D-Calif.) and former Rep. Beto O'Rourke Beto O'RourkeJimmy Carter says his son smoked pot with Willie Nelson on White House roof O'Rourke endorses Kennedy for Senate: 'A champion for the values we're most proud of' 2020 Democrats do convention Zoom call MORE were supported by 4 percent of respondents while Sen. Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-N.J.) was named by 3 percent.

All of the other Democratic candidates were named by 1 percent or fewer survey participants. Seventeen percent of respondents said they were unsure about their preference.

The results overall are largely similar to a May 17-18 Hill-HarrisX survey which also found Biden with a 19 percent lead over his Vermont-based rival. The former vice president received 33 percent support in the earlier poll while Sanders received 14 percent.

The May survey showed Warren in third place with 8 percent support and Buttigieg in fourth with 6 percent. The shift between the two candidates' support within the polls is within the surveys' sampling margins of error.

The newer poll found Biden as the top named candidate of both men and women and also across all age groups, although his lead over Sanders among voters between 18 and 34 was just one point, indicating a statistical tie. Older voters were overwhelmingly more likely to prefer the former vice president to other candidates with 47 percent of respondents 50 years old and older supporting him.

Among white voters, Biden was the preferred candidate while Sanders and Buttigieg were essentially tied for second-place. Biden enjoyed an even stronger lead among black respondents while Sanders showed strength among Hispanics although the sample size for both groups is too small to state this definitively.

Since Biden's entry into the presidential contest in April, the race has become relatively stable with no candidate able to dislodge Sanders from second-place and Biden from first, although that may change following the first debate contest which will be hosted by the Democratic National Committee June 26 and 27.

Presidential aspirants sharpened their messages over the weekend at the California Democratic Party's annual convention in San Francisco where several candidates called for impeaching President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE. Former Colorado Gov. John Hickenlooper John HickenlooperCook Political Report shifts Colorado Senate race toward Democrat Willie Nelson playing at virtual fundraiser for Hickenlooper Gardner on court vacancy: Country needs to mourn Ginsburg 'before the politics begin' MORE was booed by attendees after he expressed skepticism about a proposed Green New Deal environmental initiative and proposals to have the federal government administer medical insurance for all Americans.

Warren and Buttigieg took veiled swipes at Biden at the event with the Massachusetts senator implicitly denouncing his recent declarations that Republicans would become more inclined toward bipartisanship if President Trump is defeated in 2020. Buttigieg said that Democrats needed "a new generation of leaders," a reference to Biden's age of 76 years.

Sanders indirectly slammed Biden for skipping the event and argued that the party would not be able to defeat Trump unless it embraced a progressive agenda.

The latest Hill-HarrisX survey was conducted June 1-2 among a statistically representative sample of 1,001 registered voters. The question about 2020 Democratic presidential preference was asked of a subset of 431 respondents who said they were Democrats or independents who leaned toward the Democratic Party.

The larger survey has a 3.1 percent sampling margin of error and a 95 percent confidence level. The subset has a sampling margin of error of 4.7 percentage points.

—Matthew Sheffield