MARYLANDERS VOTED resoundingly in 2008 to permit slot-machine gambling in their state. This fall they are likely to approve an expansion that would add a sixth casino to the five authorized and allow table games such as roulette at all of them.

State officials make the case that the expansion will yield tens of millions of dollars in new revenue — a projection resting on seat-of-the-pants guesses and rosy assumptions. They are reckoning, without much evidence, that a small state can sustain one of the densest arrays of gambling venues in the nation — despite stiff competition just beyond its borders.

None of this is very persuasive. Gambling, which is sensitive to economic gyrations, is a flimsy foundation on which to build a state’s economic development program. As in other states, gambling’s promise of gushers of new money for Maryland’s coffers has almost certainly been oversold.

The best argument for expansion, therefore, comes not from state politicians in Annapolis but from local officials in Prince George’s County, where the sixth casino, an enormous one, would be located. While a glitzy mega-casino strikes us as a poor branding strategy, particularly for a jurisdiction whose image has been badly stained by corruption, it may have a local economic upside.

Officials in Prince George’s, led by County Executive Rushern L. Baker III, argue that the county has for years been starved of revenue by one of the nation’s tightest voter-imposed ceilings on property taxes. The tax cap, known by the acronym TRIM (Tax Reform Initiative by Marylanders), has been in place since 1978 and was reaffirmed by county voters in 1996. While county budgeters have lived within its strictures, it has severely limited initiatives.

While the state’s take from an expansion of gambling is a matter of some conjecture, Prince George’s, by hosting a casino, would be more assured of the shot in the arm that new revenue provides. That would help the county, a perpetual beggar, to pay its bills and fund new ideas.

Just as promisingly, it would advance the prospects of a desperately needed new regional hospital in Prince George’s, whose financing, in the formulation of State Senate President Thomas V. Mike Miller Jr. (D-Calvert), could be assured only by new gambling revenue. In our view, Mr. Miller’s use of the hospital to help secure support for an expansion of gambling smacks of extortion. But now that the expansion seems likely, let him make good on the deal.

At best, gambling is a mixed bag for Maryland — with a daunting downside in social costs. It has distorted state politics for years. Nonetheless, barring some sudden reversal in the electorate’s mood, its expansion seems all but assured. If it pays dividends for Prince George’s, that at least would be a silver lining in the cloud the issue has cast over the state.