Legend has it that Abraham Lincoln once posed a riddle: How many legs does a dog have if you count his tail as a leg? Came the answer, "Five." Replied Lincoln, "No, four. Counting a tail as a leg doesn't make it a leg."

Tell it to the sponsors of a bill to give the District of Columbia a full-fledged member of the House of Representatives. They resolutely dismiss the hurdle presented by the Constitution, which says, "The House of Representatives shall be composed of members chosen every second year by the people of the several states." Not "states and any other entities under federal control," but states, period.

The District is a unique enclave, set apart by the founders as the seat of national government. But for this purpose, the advocates assert, it is functionally no different from Maine or Montana.

The fantasy has captured many minds. The House passed a similar bill in 2007, Barack Obama has endorsed the idea and the Senate is expected to approve it this week. To maintain the current party balance, the House would expand from 435 members to 437, with Republican-dominated Utah getting an extra seat to match the one given to the Democratic-leaning District.

The bill is pretty much a sure thing to become law. But it won't banish the reality that the District is not a state and can't be treated as though it were.

Capital residents used to understand this vexing constraint. Decades ago, they wanted the right to vote in presidential elections. So they proposed and, in 1961, got a constitutional amendment to reach that end.

In 1978, Congress approved another amendment, this one to give the capital the same representation (a House member and two senators) it would have if it were a state. The measure died in 1985 after being approved by just 16 of the 38 states needed for ratification. In 1992, a similar amendment went nowhere.

The result has been intense frustration among Washington's inhabitants, who have to bear the same burdens as other Americans -- paying taxes, being subject to military conscription, enduring life without a secretary of commerce -- but have no say on such matters in Congress.

They are entitled to elect a House delegate who enjoys the same prerogatives as other members -- except the power to vote on floor legislation. Getting a House member who can't vote is like being invited to don a wet suit before entering the hot tub. District license plates carry a protest motto: "Taxation without representation."

Dissatisfied with the status quo but unable to alter it with a constitutional amendment, Washingtonians finally exclaimed: Amendment? We don't need no stinkin' amendment!

The rationale is that the Constitution, which provides for the capital, gives Congress the power "to exercise exclusive legislation in all cases whatsoever, over such District." Therefore, it may do just about anything it pleases, including give the District a vote in the House.

But the argument proves too much. The same provision gives the national legislature "like authority over all places purchased … for the erection of forts, magazines, arsenals, dock-yards and other needful buildings." If Congress can give the District a voting representative, it may give voting representatives to Fort Hood, the White Sands Missile Range and the Rock Island Arsenal. Which, obviously, it may not.

Washingtonians imagine they are victims of an injustice caused by a mere oversight, insisting that the framers never meant to disenfranchise them. Jonathan Turley, a law professor at George Washington University (location: Washington, D.C.), punctures this beguiling myth.

"The absence of a vote in Congress was clearly understood as a prominent characteristic of a federal district," he wrote last year in the George Washington Law Review. "Moreover, being a resident of the new capital city was viewed as compensation for the limitation. The fact that members would work, and generally reside, in the District gave the city sufficient attention in Congress."

A proposal by Alexander Hamilton to give the District congressional representation failed. So the founders knew what they were doing.

If Americans think they were mistaken on this point, the way to correct the error is a constitutional amendment. For the president and Congress to pretend none is needed betrays an alarming casualness about the Constitution. If they can look there and find the authority to give the District a seat in the House, they can probably also find a five-legged dog.