Did you know there is a provision in the Constitution that cannot be amended?

Yes, I’m putting on my Constitution nerd hat again today. In part because it gives me the opportunity to talk about something that has almost nothing to do with you-know who. And while my story dates from 1787, it connects with a Washington Post story published last week: “In about 20 years, half the population will live in eight states.” The piece, written by staffer Phillip Bump, is based on projections from current trends, of course, but his conclusion is totally believable.

The collective population of the eight most populous states — California, Texas, New York, Florida, Illinois, Pennsylvania, Ohio and Michigan — is growing significantly faster than the collective population of the smaller 42. (Minnesota ranks 21 in population, just below Wisconsin and above Colorado.)

After each decennial census, the map of U.S. House districts is redrawn and seats are shifted to states that gained the most population. That means that, leaving aside the gerrymandering issue, each state’s representation in the U.S. House will roughly reflect its share of population, so the delegations of the Big Eight states to the lower house of Congress will be roughly equal to the delegations of the other 42.

This is not the case in the U.S. Senate, where the representation of all states is fixed at two Senators apiece — an appropriation based on a Constitutional provision that can not ever be amended. And when I say “not ever,” I pretty much mean “forever.” That’s because Article V of the Constitution, which already establishes that an amendment requires a two-thirds vote of both Houses of Congress and ratification by three-fourths of the states, also dictates  that, “No State, without its Consent, shall be deprived of its equal Suffrage in the Senate.”

So even if 49 states agreed to move the Senate in the direction of an apportionment reflecting population, any one state could veto the move forever. (Maybe you can think of something that would convince all the smaller states to agree to give up their equal representation in the Senate, but I can’t.) So, in short, the guaranteed, perpetual equal representation of all states in the Senate must be considered a permanent feature of our system.

The arguments against the two-senator rule are fairly obvious. California, the most populous state as of the 2010 census, has 66 times the population of Wyoming, the least populous. And that difference is (almost) reflected in in California’s current 53 seats in the U.S. House, compared to Wyoming’s one seat (currently held by Dick Cheney’s daughter Liz Cheney).

But while California’s higher population is reflected in the House, nothing becomes law unless it also passes the Senate; which means the 25 states with the smallest population can block anything, despite the fact that taken together they contain less than a quarter of the whole population.

Just to torture the numbers slightly, if the senators from the 8 most populous states — representing 50 percent of the population in 20 years — favored something, but senators from the 42 smaller states opposed it, it would fail 84-16.

Things don’t usually work that way, of course. Currently, for example, the two states with the biggest population, California and Texas, rarely agree politically. In fact, Texas is the only real Republican stronghold on the list of the Big Eight (after California and Texas, they are all blue or purple states). So, at present, the equality of all states in the Senate is more beneficial to red America than blue.

That said—and in the interest of historical accuracy—the arrangement is certainly not a Republican plot. The Republican Party didn’t even exist before the 1850s. The real reason for the equality of power in the Senate was surely tied to the necessity of getting the Constitution ratified.

Under the Articles of Confederation (the loose national governing doctrine that existed before the Constitution) all states were equal on all matters. At the time of the Constitutional Convention, the population difference between the biggest state (Virginia) and the smallest (Georgia) was only about 10-to-1, compared to 69-to-1 today.

To preserve and strengthen the union, the framers needed every state to join. It’s likely that the proposal (and by “proposal” I mean the draft of the Constitution between the convention and ratification by all 13 then-existing states) would have failed if it had been seen as too advantageous to either the big or the small states. The method of awarding seats in Congress – favoring big states in the House but small states in the Senate – reeks of this political problem, or, to put it more generously, it was the solution to this problem.

When it came to appeasing the small states, the drafters had to go one step further to ensure them that they would not be overwhelmed by the big states. It’s where this post started. The framers agreed to make the guarantee of equal power in the Senate beyond even the reach of the amendment process.

The small states were giving up a lot to go along with the Constitution, and they simply had to be reassured that the new, much more powerful national government wouldn’t become the instrument by which the smalls would be dominated by the bigs.

To modern eyes, this reassurance to the small states looks pretty undemocratic. Why should the weight of the 42 smaller states be so very, very much greater in the Senate than the weight of the eight biggest? The answer is (or was): To get the deal done.