Maryland might seem a peculiar venue for a blood feud over the future of the Democratic Party. It is the second-bluest state in the United States, after Massachusetts, according to Gallup; its registered Democrats, more than 30 percent of whom are black, outnumber registered Republicans two to one. Maryland is home to an immense federal work force and is one of the states most economically dependent on the federal government. Its gun-control laws are among the strictest in the nation. In 2012, Maryland, Maine and Washington became the first states to ratify same-sex marriage by popular vote. Barack Obama’s statewide margin of victory was roughly 26 points in 2008 and 2012, the fifth highest in the United States. The last time the G.O.P. won control of the Maryland State Legislature was in 1897. So reliable is its party affiliation that, as a Democratic senator’s chief of staff puts it, “If Maryland ever becomes a jeopardy state, then the whole thing is gone.”

This past March, when Barbara Mikulski, the longest-serving woman in United States Senate history, unexpectedly announced that she would not be seeking a sixth term in 2016, national progressive groups quickly threw their weight behind their dream candidate: Donna Edwards. A pugnacious former community organizer, Edwards is a four-term African-American congresswoman from Prince George’s County, one of the most affluent majority-black counties in the United States. But she wasn’t the favorite of establishment Democrats. For them, the obvious choice to replace Mikulski was the seven-term congressman Chris Van Hollen, who is considered a progressive like Edwards, but has a reputation for coolheaded practicality and for working well with Republicans. Of the bills sponsored by Van Hollen in the previous session of Congress, 37 percent included at least one Republican co-sponsor. For Edwards, the corresponding figure was 0 percent. Where she is viewed as a warrior for liberal causes, he is seen as a conciliator, one whose let’s-sit-down-and-talk-this-over geniality led to his tenure as chairman of the Democratic Congressional Campaign Committee from 2007 to 2011 and, thereafter, to his designation as the House Democrats’ point man on bipartisan budget discussions. As their lead negotiator, Van Hollen has immersed himself in the sort of legislative sausage-making that typically entails compromise, like his expressed willingness, in 2012, to consider restructuring Social Security as part of an overall deficit-reduction agreement. To progressives, this was nothing less than apostasy.

Though the Senate Democratic primary was a year away, the national groups supporting Edwards knew that Van Hollen would be viewed as the prohibitive front-runner if they didn’t define the stakes of the contest immediately. Three of these groups — the Progressive Change Campaign Committee, Democracy for America and Blue America — sent out a blizzard of fund-raising solicitations, petitions and emails to members and to the media, one of which hailed Edwards as “a true Elizabeth Warren Democrat,” referring to the U.S. senator from Massachusetts whose confrontational stances on economic issues have galvanized the left. Van Hollen received an altogether different reception. Within hours after he made his candidacy official on March 4, three other voices from the liberal wing of the party — MoveOn, Credo Action and Daily Kos, the website run by the activist Markos Moulitsas — openly questioned his progressive bona fides and implied that he was one of a breed of “corporate ‘New Democrats.’ ” Moulitsas’s website declared that Van Hollen’s flexibility on Social Security amounted to “a major red flag,” making him “a candidate that may bargain away retirement security.” Edwards, meanwhile, entered the race pointedly pledging never to tamper with Social Security, “no ifs, ands, buts or willing-to-considers.”

Whichever Democratic candidate wins the primary next spring, he or she will be heavily favored to become the state’s next U.S. senator. Because of this, the Maryland contest is unlikely to hinge on which candidate can appeal to the broadest spectrum of voters on Election Day. Rather, it will be a fight over what a true Democrat should, and should not, be. This identity struggle was born out of devastating losses at the polls last November, but it is rooted in intraparty disagreements that have been decades in the making. And it is by no means limited to Maryland. In Illinois, Florida, California and elsewhere, progressive groups have asserted their energies to promote what they hope will be populist warriors in the Elizabeth Warren mold while weeding out those judged to be ideologically tepid. The recently announced presidential candidacy of Bernie Sanders, the self-described socialist senator from Vermont, will no doubt increase the party’s gravitational pull leftward. Still, given Van Hollen’s history as a highly effective liberal legislator, the effort to push him aside in favor of Edwards is a striking development for a party that has largely kept its internal skirmishes under wraps. As Neil Sroka, the communications strategist for the liberal group Democracy for America, puts it, “We view primaries like this one as a fight over the future of the Democratic Party.”