File: Sen. Elizabeth Warren (D-Mass.) (AP Photo/Steven Senne)

Sen. Elizabeth Warren (D-Mass.), whose economic populism has made her a hero to the left, will headline this summer's Netroots Nation, an annual gathering of liberal bloggers and activists.

Warren will deliver a keynote address at the progressive confab, which has become an important stop on the Democratic circuit and will be held July 17-20 in Detroit. In the draft of an e-mail to be sent to supporters later on Tuesday, Netroots Nation executive director Raven Brooks calls Warren "one of our country's biggest progressive champions."

Organizers invited former secretary of state Hillary Rodham Clinton to speak as well, but Clinton's office declined the invitation, according to Netroots Nation communications director Mary Rickles.

Clinton, a prospective 2016 presidential candidate, has largely avoided appearing at overtly political events since stepping down from the State Department early last year. Clinton's memoir, "Hard Choices," is due out June 10, and she is not expected to return to the political stage to campaign for Democratic candidates in the midterm elections until after her book publicity tour.

This will be the third appearance at Netroots Nation for Warren, who says she is not interested in running for president. Warren spoke at the conference in 2010, while she was advocating for the newly-created Consumer Financial Protection Bureau, and again in 2012, when she was a candidate for the Senate.

Warren is scheduled to speak the morning of July 18, coming after this month's release of her memoir, "A Fighting Chance." In his e-mail, Brooks writes that Warren's book "shows why she has chosen to fight tooth and nail for the middle class."

This summer's Netroots Nation, the ninth annual gathering, will feature speeches, panels and workshops focused on the midterm elections and progressive economic themes, including fighting against corporate influence, Rickles said. The group is planning more than 80 panel discussions. Rickles highlighted the diversity of the accepted panelists; 53 percent are people of color while 61 percent are women, she said.

Netroots Nation is a gathering of bloggers, labor leaders, social justice advocates, grassroots organizers and other progressive leaders. Past conference headliners have included former president Bill Clinton, former vice president Al Gore, House minority leader Nancy Pelosi (D-Calif.) and Senate Majority Leader Harry Reid (D-Nev.). In the run-up to the 2008 presidential primaries, Netroots staged a candidates forum featuring seven leading contenders, including then-Sen. Barack Obama (D-Ill.) and then-Sen. Hillary Clinton (D-N.Y.).