The map of Maryland’s 3rd Congressional District , as redrawn by Democrats in Annapolis after the most recent U.S. Census, is nothing if not cartoonish. Comically gerrymandered, it slices and dices counties, communities and neighborhoods. Splattered east, west, north and south of Baltimore, it also takes pains to hack the city itself into pieces. As a case study in majority-party abuse, Maryland’s 3rd District has few peers nationally.

In fact, Maryland itself now counts as the most — read: worst — gerrymandered state in America. In a (not yet published) definitive study of congressional districts conducted by a nonpartisan geospatial analysis firm called Azavea, the average compactness of Maryland’s eight congressional districts ranks dead last among the 50 states, as computed by an average of four measures of compactness.

Maryland’s 3rd District comprises parts of Montgomery, Anne Arundel, Howard and Baltimore counties, as well as pieces of the city of Baltimore — areas widely separated, in some cases by water. Forty-two of the district’s precincts are split, severed or dissected by the map’s crazy-quilt demarcations. It is not only the state’s most extreme example of cartographical shenanigans; it also ranks as the third-least-compact district in the country. But Maryland’s 6th, 2nd and 1st Districts aren’t far behind; each is among the 25 worst nationally.

Driven by the prospect of adding a seventh seat to the six their party controls in the eight-member House delegation, Democratic Party leaders went overboard in carving up territory so that Democratic votes would be deployed to maximum advantage, even if it meant stitching them into districts resembling violently spilled coffee. The results are districts of diverse and, in some cases, wildly incongruent interests that will be all but impossible for a single representative to navigate, let alone fairly represent on Capitol Hill.

Maryland’s chief executive, Gov. Martin O’Malley (D), was among the chief authors of this disgrace. Partisan affiliation notwithstanding, anyone with a remote interest in good government, Republican or Democrat, can only be appalled by the handiwork of Gov. Gerrymander and his hyperpartisan assistant draftsmen.

Such blatant gerrymandering results in less competitive, or utterly non-competitive, races for Congress, leaving the only real competition in party primaries. When hard-core party loyalists, rather than a fair sampling of all voters, are calling the shots, the winners have little incentive to embrace moderation or strike bipartisan deals. That helps explain the current, hyperpartisan makeup of Congress and the resulting paralysis on critical national questions.

Fortunately, Maryland voters have a chance to reject this map in an up-or-down vote on Election Day and send it back to Annapolis for redrafting. On ballot Question 5, they should vote against the law that establishes the district lines. Doing so will send a message to Mr. O’Malley and other party leaders that in America, voters should be able to choose their candidates, not the other way around.