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The answer is the Congressional Baseball Game, a tradition among lawmakers going back to the early part of the 20th Century that in recent years has raised $600,000 for charity.

"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.

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.

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.

These are generally lighthearted affairs, as CNN's Chris Moody found when he attended back in 2015 and saw President Obama cheering against Paul, who was then just getting ready to mount a bid for the Republican presidential nomination.

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The Congressional baseball game is a long-running, bipartisan tradition - CNN