WASHINGTON (CNN) Joe Biden has expanded his edge over the Democratic field in a new CNN poll conducted by SSRS , with 29% of Democratic and Democratic-leaning registered voters saying they back the former vice president.

That's up 7 points compared with a late June CNN survey. No other candidate has made meaningful gains over that time.

The shift returns Biden to a double-digit lead over his nearest competitors, Vermont Sen. Bernie Sanders at 15% and Massachusetts Sen. Elizabeth Warren at 14%. Their support is largely unchanged since earlier this summer.

Aside from Biden's increase, the only statistically meaningful change in the candidate standings is a 12-point decline in support for California Sen. Kamala Harris , who stood at 17% support in the June poll but now has the backing of 5% of potential Democratic voters. That's similar to the level of support she had in the spring before a surge following her initial debate performance.

The June CNN poll was conducted in the days immediately following the first round of Democratic National Committee-sanctioned debates among the presidential candidates. Biden's performance in that debate was widely panned, while Harris had a standout moment challenging him over his views on school busing integration. Biden's numbers have rebounded since then in other polling as well, and Harris's have returned to roughly where they were pre-debate.

Biden's advantage in the poll is boosted by stronger support from self-identified Democrats (31%) than from independents (23%), older voters (34% among those age 45 and older) than younger ones (23% among those under age 45) and from moderate and conservative voters (34%) than liberals (22%).

Among liberals, in fact, the race is a near three-way tie: 23% choose Warren and 22% each back Biden and Sanders. No other candidate even hits 5% among this group.

Biden is also bolstered by those voters who say their top priority is choosing a candidate with a strong chance of beating President Donald Trump, a group that makes up 54% of Democratic and Democratic-leaning registered voters (35% in that group support Biden). That group has shrunk slightly since June, when 61% said defeating Trump outweighed nominating a candidate who "shares your positions on major issues."

While Democrats do not appear sharply divided on this question by ideology (53% of liberals and 57% of moderate or conservative Democratic voters say beating Trump is more important), race (54% among whites and non-whites prioritize beating Trump) or partisanship (52% of self-identified Democrats prioritize beating Trump as do 59% of Democratic-leaning independents), there are notable divisions by age and by education among whites, which highlights the challenge facing candidates when they attempt to appeal to those on both sides of this line.

Among those under age 45, 56% say it's more important to nominate a candidate who shares your positions on issues, while 66% of those age 45 and older say it's more important to nominate someone who can defeat Trump. Among whites without college degrees, 49% say it's more important to nominate someone who shares their view on issues while 45% say beating Trump is more important, but among whites with a college degree, 65% say it's more important to beat Trump.

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The poll -- which is among those used by the DNC in order to determine which candidates will qualify for the debate stage in September -- marks the fourth such qualifying poll in which former HUD Secretary Julián Castro has hit 2%, securing his spot on the stage in Houston next month.

Castro is the 10th candidate to qualify for a spot. Two other candidates say they have already met the fundraising threshold for appearing on stage in September, but have not yet hit the polling mark: Businessman Tom Steyer remains one poll shy of qualifying and Hawaii Rep. Tulsi Gabbard now has two after landing at 2% in this poll. She needs two more to make the debate stage.

Washington Gov. Jay Inslee's campaign said Monday that he had hit the 130,000-donor threshold. Additionally, author Marianne Williamson's campaign announced Tuesday that she had met the fundraising threshold.

Neither Inslee nor Williamson has hit 2% in any qualifying polls.

New York Sen. Kirsten Gillibrand has hit 2% in one qualifying poll and is the only other active candidate to have hit that mark in any qualifying poll, but she has not met the fundraising criteria.

Asked which candidates they'd most like to hear more about, Democratic and Democratic-leaning registered voters are largely focused on the people who will appear on September's debate stage.

The 10 candidates named by 5% or more of respondents have all now qualified for inclusion in the debates. Warren now tops this list, named by 20%, despite a slight decline in interest in hearing more about her compared with June. She is followed by Harris at 18%, Sanders at 16%, Biden at 15%, Buttigieg at 13%, Booker and O'Rourke at 10% each, Yang at 6% and Castro and Klobuchar each at 5%.

The CNN Poll was conducted by SSRS August 15 through 18 among a random national sample of 1,001 adults reached on landlines or cellphones by a live interviewer. Results for the full sample have a margin of sampling error of plus or minus 3.7 percentage points, for the subsample of 402 Democrats and Democratic-leaning independents who are registered to vote, it is 6.1 points.