Hillary Rodham Clinton speaks during the 2015 United States Conference of Mayors on May 20, 2015 in San Francisco, California. (Photo by Justin Sullivan/Getty Images)

Hillary Rodham Clinton is skipping a gathering of politically active progressives next month that would have put her on the same stage with her Democratic challengers -- and likely set up unwelcome comparisons with liberal heroine Sen. Elizabeth Warren (D-Mass.)

Clinton sent regrets for Netroots Nation, a three-day political conference that is a draw for some of the most ardent progressive activists, because she has previously scheduled speaking events in Iowa and Arkansas, her campaign said.

"Our campaign looks forward to earning the support of the Democrats participating in this conference but Hillary Clinton has scheduling conflicts which will prevent her from attending,” campaign spokesman Jesse Ferguson said. “She wishes them the best on their conference."

Clinton is far and away the Democratic front-runner, with 75 percent of Democratic support in a Wall Street Journal/NBC News poll out this week. But she has struggled to gain the enthusiastic backing of the far left, despite running a very left-leaning campaign so far.

Sen. Bernie Sanders (I-Vt.), whose strongly progressive campaign platform and straight-ahead style is drawing large crowds, will attend the conference in Phoenix. So will former Maryland Gov. Martin O’Malley, who has tried to capitalize on any far-left disaffection by campaigning as a progressive champion and calling Clinton late to the cause.

Sanders and O’Malley will participate in a candidate forum that Clinton was also invited to attend, Netroots spokeswoman Mary Rickles said. Sanders captured 15 percent support in the same WSJ/NBC poll, and O'Malley 2 percent. That suggests Sanders is heir to much of the progressive fervor for Warren, who disappointed some on the left by declining to challenge Clinton for the 2016 nomination.

Warren will also attend the conference. Her fire-breathing attacks on Wall Street made her one of the stars of last year’s Netroot Nations, and she draws large and enthusiastic crowds wherever she speaks. Although she has made no move to reconsider her decision to stay out of the race, Warren still poses a hazard for Clinton. The zeal of Warren's supporters points up the perception, still prevalent on the left, that Clinton is a privileged intimate of the very wealthy.

“I don’t know what her schedule planning is, but I certainly hoped that she would make it,” Rickles said, noting that both Sanders and O’Malley will attend the same Iowa Democratic Party event as Clinton on July 17.

Clinton, however, is committed to an Arkansas Democratic Party dinner on July 18, the day of the scheduled candidate panel.

Asked whether Clinton might be staying away at least in part because she may not feel welcome, Rickles said no. Attendees include some Democrats who support Clinton and some who do not, she said.

“Our people want to hear from her and ask her questions. I expect there will be some attendees who are disappointed she is not there,” Rickles said. “We would welcome her if she’s able to change her schedule.”

Clinton has attended a Netroots event once in 2007. She and then-Sen. Barack Obama were among the Democratic prospects who participated in a similar candidate forum ahead of the 2008 primaries, Rickles said.

About 3,000 activists are expected to attend the 2015 convention.