The race for the Democratic nomination is already being described as a battle of the party establishment against its progressive left wing.

Rep. Donna Edwards announced Tuesday she would run for the seat being vacated by longtime Sen. Barbara Mikulski of Maryland, becoming the second candidate in what is likely to be one of the most competitive Democratic primaries of the 2016 election cycle.

In a video posted to her new campaign website, Edwards, D-Md., highlighted her progressive record as a defender of unions and the middle class, entitlement programs, and women’s and minority rights.

"See, I've lived the American dream, a middle-class American dream, the one you have to work hard for just to hold on to, the one that's slipping away for far too many Maryland families," she said in the video. "These are the people I fight for."

Edwards, who was elected to Congress in a 2008 special election, will take on fellow Rep. Chris Van Hollen in a party primary that most expect will only get more competitive in the next several weeks.

Mikulski, the longest serving woman in the history of the U.S. Congress, announced her intentions last week, catching most in the state party off guard. Her retirement will mark the first time one of Maryland’s Senate seats will be open since 2006, when Sen. Ben Cardin, also a Democrat, was elected. The overwhelmingly Democratic state has not elected a Republican to the Senate since Charles Mathias, Mikulski’s predecessor, in 1980.

Seven of the state’s eight sitting members of the House of Representatives are Democrats, and at least four – Reps. John Sarbanes, John Delaney, C.A. “Dutch” Ruppersberger and Elijah Cummings – have suggested they may be interested in vying for the seat. Former NAACP President Ben Jealous, Baltimore Mayor Stephanie Rawlings-Blake and former Lt. Gov. Kathleen Kennedy Townsend are also frequently named as possible contenders. Former Gov. Martin O’Malley, who has shown signs of preparing a presidential campaign, has bowed out.

“There are a lot of candidates that are building up reputations and experience from the county level and the state level, and ultimately the federal level,” says one Democratic strategist who spoke on the condition of anonymity because of the possibility of working for one of the candidates in the race. “If you pass [on running this year], you don’t know if you’re going to have another shot because of how deep the bench is.”

Despite losing the governor’s mansion in a shocking upset in 2014, Democrats say they are confident they will hold not only Mikulski’s seat, but the seats of any of the candidates who must give up their House seats in order to run for Senate. Instead, the race is being described as a battle of the party establishment versus the progressive left wing, not unlike the battle some hope to see between Hillary Clinton and Elizabeth Warren for the presidential nomination.

But those with close ties to state politics say the effort by groups such as EMILY’s List or the Progressive Change Campaign Committee to draw Van Hollen as an establishment, rather than a progressive, Democrat will ring false.

“People in Maryland don’t see Chris Van Hollen as the moderate Democrat,” says Democratic strategist Oscar Ramirez, who worked for the state party under O’Malley, as well as stints on Capitol Hill and two presidential campaigns. “These are two good progressive candidates that have different styles.”

“National groups want local races to be proxies,” he says, but “we know he is a real progressive voice.”

Edwards likely faces an uphill battle nonetheless. Although she has the backing of progressive advocacy groups such as EMILY’s List, PCCC and Democracy For America, Van Hollen has stronger ties to the national party. He formerly served as the chairman of the Democratic Congressional Campaign Committee and is considered a fundraising powerhouse. He received the endorsements of Senate Minority Leader Harry Reid, D-Nev., and state Attorney General Brian Frosh.

Stella Rouse, the assistant director of the Center for American Politics and Citizenship at the University of Maryland, says while Edwards may only be able to put so much distance between herself and Van Hollen as a progressive, potentially having a woman at the top of the presidential ticket could boost her candidacy.

Especially with Mikulski leaving, Rouse says, Edwards “could make the case for keeping the seat for a woman.”

That goes double if Warren at least stays in the presidential conversation.

“That element would help somebody like Edwards because she’s from the same wing of the party that Warren is,” Rouse says. “Warren could bring a spillover effect.”

Fundraising will go a long way in determining the nominee, but ultimately, Ramirez says, much will come down to how well the candidates are able to turn out their bases of support. According to the 2010 Census, nearly 30 percent of Marylanders are African-American, a group that largely stayed home in the 2014 midterms but has been successfully mobilized in the past.

Ultimately, party insiders believe state Democrats will be motivated to unite behind whichever candidate is nominated, potentially keeping the primary from turning into a mudfest even though they’ll “do what they need to do to win.”

“They’re very interested in seeing that seat stay Democratic, so if they start cannibalizing each other, that could be an opening for a Republican,” Rouse says.

If elected, Edwards would be the first African-American to represent Maryland in the U.S. Senate.