Put it in the category of things we know for sure that just ain't so.

No sooner did the Democratic National Committee announce it had chosen Philadelphia, Pa., as its 2016 convention site than a lot of us political analyst types popped out the conventional wisdom about "appealing to a swing state in the general election."

It sounds good and it makes sense, as far as it goes. It just doesn't go very far.

Sure, it ought to help the Democrats to have their convention in a state that they absolutely have to win in November. It also ought to help the GOP to have its convention in Cleveland; no Republican has ever won the White House without carrying Ohio.

But let's face it. If the Democrats do win Pennsylvania, it won't be because they had their convention in Philadelphia, which is already a motherlode of Democratic votes. And if the Republicans wind up winning Ohio, it won't be because they won over a lot of precincts in Cleveland, which is a similarly rich trove of Democratic support in elections at all levels.

The idea that conventions are located with an eye toward winning the host city's state is popular to the point of being irresistible. But it doesn't fare well against the facts.

True, both parties took their conventions to swing states in 2012. But both parties wound up losing those swing states. The Republicans headed for Tampa in pivotal Florida but their nominee, Mitt Romney, lost the state in November. The Democrats went to Charlotte, N.C., in part to celebrate winning there in 2008 (for the first time in 32 years). But four years later, after holding their convention in the Tar Heel state, they saw it go GOP.

In a sense, both parties thought they were wooing swing states in 2008, too. The Democrats saw Colorado as winnable, even though they had only won it three times since the 1930s. The Republicans went after Minnesota, which had the longest streak of voting blue for president in the whole country. The Democrats managed to make their swipe, the GOP didn't even come close.

But to the two Obama elections, the parties' choice of their convention sites seems to have had only occasional connection to the voting patterns of the states. From 1988 to 2004, the parties sited 10 conventions and chose a swing state only once (the GOP went to Philadelphia in 2000). The rest of the time they were in states as reliably red as Texas and Louisiana or as true blue as Massachusetts and California.

In fact, through those five cycles, the Republicans twice went to states (California and New York) that they would lose by double digits in the fall voting for president. The Democrats, for their part, took a similar drubbing in Georgia after convening in Atlanta in 1988. It is hard to imagine that party professionals in either case really thought things would be different because of the convention.

In the cycles where the two parties did manage to win their convention-site states, they held those gatherings in states they could scarcely have lost – such as Texas for the GOP and Massachusetts for the Democrats.

The truth is, political parties locate their conventions much the way large trade associations and professional groups do. They look for geographical balance (which is why the first generations of conventions were usually held in Baltimore and later generations in Chicago). They look for fun stuff to do (see New Orleans, for example, or San Diego). But in the end, it comes down to state-of-the-art facilities, an adequate supply of quality hotel rooms and a financial aid package from the city and private donors.

Those criteria make a lot more sense than a hopeful lunge after an iffy package of electoral votes, especially given the poor return on past attempts.

Going back to the 1950s, and the last 30 choices of convention sites, the party has lost the state where it held its convention 16 times and won it 14.

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