Rep. Donna Edwards (D-Md.) is flanked by House Minority Leader Nancy Pelosi (D-Calif.), left, and Rep. Steve Israel (D-N.Y.) as she speaks to reporters after Pelosi announced that Edwards will become co-chair of the House Democrats’ Steering and Policy Committee with Rep. Rosa DeLauro (D-Conn.) during a news conference at the Capitol on Nov. 17, 2014. (J. Scott Applewhite/AP)

It has been one week since five-term Sen. Barbara Mikulski (D-Md.) announced her impending retirement, and already the race to replace her has the makings of a battle royal — one with ramifications beyond Maryland.

Rep. Donna F. Edwards is expected to enter the race this week, The Washington Post reported late Sunday, setting up a primary battle against Rep. Chris Van Hollen, who has been on the fast track to national prominence since defeating a veteran Republican lawmaker in 2002.

The Democratic primary field is certain to grow, with Baltimore-area candidates such as Rep. John Sarbanes, Rep. C.A. Dutch Ruppersberger, Rep. Elijah E. Cummings or Baltimore Mayor Stephanie Rawlings-Blake possibly joining the D.C. suburban hopefuls. But with Edwards and Van Hollen staking early claims, both of them formidable campaigners, the Maryland Senate primary is shaping up to be the proxy war over the direction of the Democratic Party that seems unlikely —at this point — to play out in the presidential race.

E-mail scandal aside, Hillary Clinton still occupies a commanding position entering the 2016 primary derby. The candidate Clinton is seen as most vulnerable to on the left, Sen. Elizabeth Warren (D-Mass.), insists she isn't interested in a 2016 presidential run. And while former Maryland governor Martin O'Malley has made some noise about pushing Clinton from the left — decrying Clintonian "triangulation" in Iowa and calling Friday for the restoration of Glass-Steagall banking law, dismantled in 1999 under President Bill Clinton — his campaign posture at this point is far from that of a firebrand. The only other Democrat openly exploring a presidential run, former Virginia senator Jim Webb, is an avowed centrist with limited appeal to the party's progressive wing.

The tension between mainstream and progressive Democrats is on display elsewhere in the country — including in the pending Chicago mayoral race, where incumbent Rahm Emanuel is facing a vigorous challenge from the more liberal Jesus "Chuy" Garcia — but when it comes to 2016, the battle may play out most clearly in Maryland,

If there ever was a candidate progressive Democrats could embrace, it's Edwards. She came to prominence as a community advocate and the leader of national activist groups on issues such as domestic violence, campaign finance reform and the environment. In the House, she has staked a claim as a leader of the caucus's leftmost faction, with National Journal in 2013 naming Edwards one of the seven most liberal members of the House.

As a Senate candidate, she'll be able to stake out clear distinctions with Van Hollen on fiscal issues, having taken a hardline approach against Republican budget cuts. For instance, Edwards was one of the 95 Democrats who voted in 2011 against the budget deal that resulted in the drastic "sequestration" cuts, while Van Hollen — by then ranking member of the House Budget Committee — voted for it.

Van Hollen is by no means a conservative or even a moderate. He holds perfect or close-to-perfect ratings from liberal groups including the AFL-CIO, NARAL Pro-Choice America, the Human Rights Campaign, the NAACP and the National Council of La Raza. But as a mainstream Democrat with House leadership ambitions, he has taken some stances at odds with ideological purity that could leave him vulnerable in the Democratic primary race.

That tension was on display Friday, when Van Hollen appeared on WAMU-FM to talk about his Senate run and spent a good deal of time explaining why he was sufficiently progressive for Maryland Democrats — highlighting, in particular, his role as the chief Democratic foe to Republican budget whiz Rep. Paul Ryan (R-Wis.).

"If you look at my record, I'm a very progressive candidate," he said.

But he also faced tough questions from left-leaning callers, and curious about his past statements on the Simpson-Bowles plan, which included Medicare and Social Security benefit cuts, calling it a "framework" for deficit reductions and on whether he supported the Obama administration's efforts (since abandoned) to nominate Wall Street investment banker Antonio Weiss as an high-ranking Treasury official — a flashpoint for progressives.

Said Van Hollen, "I'm against cutting Social Security; I'm against cutting Medicare." And on Weiss's nomination, he said, "I agreed with Elizabeth Warren. ... I thought it was important to signal that we're not going to draw people from Wall Street. ... We're going to hold Wall Street accountable."

But he also laid out the framework of another campaign pitch: Chris Van Hollen may not be the most ideologically pure liberal in the race, but he's the one best positioned to get things done.

"I believe in a progressive agenda, and I believe in being effective at getting that progressive agenda through," he said. "I think that when you look at candidates, you have to look both at their values and their ability to translate those values into action and their ability to prevent, in this case, House Republicans or Senate Republicans from rolling back the important progress we've made."

This item was updated 3/10 to include more details on Van Hollen's record.