Rep. Donna Edwards, D-Md. speaks on Capitol Hill in November. (AP/J. Scott Applewhite)

PHOENIX — Rep. Donna Edwards on Thursday brought her campaign for Maryland’s open U.S. Senate seat to a nationwide gathering of progressive activists, where she asked for their help in promoting her candidacy and creating a more diverse chamber.

“You tell them to just make room for one black woman in the United States Senate,” Edwards told a few hundred activists during what was billed as the keynote address on the opening night of the Netroots Nation gathering here. She noted that there has not been a black woman in the U.S. Senate since Carol Mosely Braun, a Democrat who represented Illinois from 1993 to 1999.

Edwards, who is from Prince George's County, faces a tough Democratic primary against Rep. Chris Van Hollen in her quest to take over the seat of retiring Sen. Barbara A. Mikulski (D-Md.).

Thus far, Van Hollen, who is from neighboring Montgomery and has much deeper support among the Democratic establishment, has raised nearly three times as much money for the race as Edwards.

[Van Hollen far ahead of Edwards in race for campaign cash]

Edwards, who worked as a community organizer before joining Congress, has a long history with the Netroots Nation conference, having attended its first gathering a decade ago.

Activists from the conference provided financial and organizational help in 2008 when Edwards was first elected to the House, after knocking off 15-year incumbent Albert Wynn in a Democratic primary. And progressive groups, such as Emily's List, continue to be among her strongest backers.

“This is her base,” said Jim Dean, chair of Democracy for America, among the liberal groups represented at the conference that have urged Edwards to run as Mikulski's successor. “I think she’s got a very credible pathway to victory. The folks who come out to vote in Maryland primaries are within her reach.”

[Edwards, leading other women into politics]

Much of Edwards’s speech focused on challenges facing the African-American community, including gun violence and police mistrust, as well as income inequality.

Edwards, a single mother, said that if elected to the Senate she would represent “Jamal’s mother,” a reference to a line in President Obama’s eulogy last month for the Rev. Clementa Pinckney, who was among nine African-Americans killed in a Charleston, S.C., church. Speaking out against racial bias, Obama talked about the need to overcome “the subtle impulse to call Johnny back for a job interview but not Jamal.”

[The black president some worried about has arrived]

In her remarks, Edwards also decried the influence of the National Rifle Association, which she called “nothing more … than a lobby for corporate gun interests.”

“We’re going to fight them until they’re done,” she said.

Edwards’s appearance here fell on the same day that Van Hollen challenged her to sign a pledge intended to keep outside groups from spending money on the race.

Edwards declined, saying in a statement that “until we have real reform, it is wrong to silence … pro-choice Democratic women, working families and progressive advocates in this campaign.”

Under Van Hollen’s proposal, if a third-party group aired any ad or contacted voters in support of one candidate or attacked the other, the candidate who benefited from the spending would make a donation to charity that was equivalent to half of the amount spent.

“We can discourage outside groups and deep pocketed individuals from dumping money into Maryland,” Van Hollen wrote in a letter to Edwards. “This should be a race for Marylanders.”

Edwards rejected the pledge, saying in a statement that “until we have real reform, it is wrong to silence ... pro-choice Democratic women, working families and progressive advocates in this campaign.”

Rachel Weiner contributed to this report from Washington.