By Chris Cillizza

CNN Editor-at-large

(CNN) -- For most people, their first thought upon hearing of the shootings at a baseball practice for Congressional Republicans went something like this: Why are members of Congress on a baseball diamond at 7 a.m. on a weekday?

The answer is the Congressional Baseball Game, a tradition among lawmakers going back to the early part of the 20th Century.

"It's one of the best things we do," Sen. Rand Paul, who was at the practice when the shooting broke out, told CNN Wednesday morning.

"Every year, with a few interruptions, Senate and House members of each party team up to settle scores and solidify friendships off the floor and on the field," according to the game's website. "Members usually sport the uniform of their home states and districts, and although proportional representation is not required, elected officials of many states play to win every year."

In the run-up to the game, which is scheduled to be played tomorrow at Nationals Park, both sides -- Republicans and Democrats -- hold early-morning practices in neighborhoods around Washington.

"There's not much scheduling going on up here at 6:30 in the morning, so in terms of practice, there's no excuses," freshman Kansas Rep. Roger Marshall, a Republican, told Roll Call in a preview piece on the game.

As news of the shooting at the Republican practice broke, Democrats were preparing to practice too. "Thoughts and prayers w my Republican baseball friends this morning," tweeted Colorado Rep. Jared Polis. "Dem practice canceled holding in dugout w security."

The game began in 1909, pushed by Rep. John Tener of Pennsylvania, who once played pro baseball for the Baltimore Orioles, among other teams. It continued, generally unbroken, until 1958 when Speaker Sam Rayburn ended it because the game had grown "too physical," according to the game's official website. Four years later, Speaker John McCormack re-started the game -- and began its association with the Roll Call newspaper.

Over the past two decades, the game grew into a bigger and bigger deal. When I covered the game for Roll Call in the early 2000s -- I attended the game as a fan several other times -- it was played in Prince George's County at the home of the Bowie Baysox. It moved to RFK Stadium in Washington, DC for a brief period in the 2000s. The game has been at Nationals Park since 2008.

Democrats had dominated for years thanks to the golden arm of Rep. Cedric Richmond, a Louisiana Democrat. Republicans broke that streak in 2016 when they claimed an 8-7 victory.

Check out this piece from The Atlantic from 2013, "The fiercest battle in DC is on the baseball diamond."