Del. Joseline Peña-Melnyk (D) is running to represent Maryland’s 4th Congressional District in the House. (Marvin Joseph/The Washington Post)

THE DEMOCRATIC primary in Maryland’s 4th Congressional District, which includes big chunks of Prince George’s and Anne Arundel counties, features several well-known candidates whose claims to the job include serious credentials, broad experience and real achievements. The best of them, by a good margin, is state Del. Joseline Peña-Melnyk.

Ms. Peña-Melnyk, a former federal prosecutor who has served as a state lawmaker since 2007, lacks the name recognition of her main rivals for the nomination, former lieutenant governor Anthony G. Brown and former Prince George’s state’s attorney Glenn Ivey. For that, she more than compensates with preternatural endowments of energy, grit and determination.

Those qualities have drawn rave notices practically from the moment she arrived in Annapolis, representing a district in Prince George’s, as a freshman legislator. They account both for her impressive legislative record and for her tireless service to constituents. By a variety of accounts, no one in the state legislature works harder.

That matters in a race in which the most viable candidates, liberals in one of the nation’s most left-leaning congressional districts, are nearly indistinguishable in terms of policy and ideology. All favor expanding an array of government programs and services; none has a detailed explanation of how to pay for it. Any would make a plausible successor to the incumbent, Rep. Donna Edwards, who is running in the Democratic primary for U.S. senator.

Ms. Peña-Melnyk , born in the Dominican Republic, grew up in an impoverished immigrant household and had the pluck to make her way through college and law school. She won a seat in the legislature without the support of Prince George’s local Democratic bigwigs, but quickly made a mark as a driven, detail-oriented workhorse, pushing through a variety of substantive bills and impressing colleagues who regarded her as an outsider.

She played a key role in patient-friendly legislation requiring the digitization of medical records in Maryland, the first state to enact such a mandate, which was initially opposed by insurance companies. And she was largely responsible for a fair-minded bill that protects the rights of urban areas of the state by designating prison inmates according to their home addresses, not the location of the facilities where they serve time.

Ms. Peña-Melnyk’s chief opponents, also attorneys, are accomplished former elected officials, and a credit to the deep bench of political talent in Prince George’s. Mr. Ivey, who was the county’s top prosecutor for eight years, knows Capitol Hill well — he was once a senior congressional aide — and is respected for his work on domestic abuse. Mr. Brown, a former state lawmaker and Iraq War veteran, served two terms as lieutenant governor before he ran unsuccessfully, in 2014, to succeed Gov. Martin O’Malley. Both men, graduates of Harvard Law School, are astute and knowledgeable.

Yet it is hard to imagine that either would be as effective or independent-minded in Congress as Ms. Peña-Melnyk, a doer with a knack for making things happen. We endorse her in the April 26 Democratic primary.