By Susan Cornwell

WASHINGTON (Reuters) - Democrats and Republicans take their age-old rivalry from the halls of Capitol Hill to the open air of a professional sports stadium on Thursday for an annual congressional baseball game whose stakes could hardly be higher.

Since 1909, when Democrats and Republicans began a tradition of donning gloves and playing each other once a year, the two sides have won an equal number of games, the organizers' website says.

In a city obsessed with massaging and spinning information, some tried to dampen expectations ahead of the game.

"I haven't played since grade school and I was terrible there," said Republican Mick Mulvaney, who started playing shortstop at the annual event two years ago. "I haven't completely embarrassed myself yet."

    The 2015 congressional baseball game will be played on Thursday evening at the Washington stadium normally used by Major League Baseball's Washington Nationals.

In an age of partisan division when many lawmakers fly home every weekend, the game is one of few congressional traditions left from years ago when lawmakers were a more social bunch.

"Everybody's always running to an airport. There are so few opportunities where you're able to just engage your colleagues without suits on and debating," Democratic Representative Mike Doyle of Pennsylvania said in a C-SPAN TV interview on the game.

A key to this year's matchup will be Democratic pitcher Cedric Richmond's shoulder. The Louisiana representative, who pitched in college, has anchored the team in recent years.

"I am starting on the mound; I don't know how long I can go," Richmond told Reuters in a Capitol lobby, his 13-month-old son on his shoulder.

Richmond said if he can't go all nine innings, Representative Patrick Murphy of Florida will go in as Democratic pitcher.

Mulvaney put the odds in the Democrats' favor. "The Democrats will probably win because good pitching dominates. But we're putting the best team we've put on the field since I've been here," said the South Carolina congressman.

When told of reports that Richmond had a bad shoulder, Mulvaney said, "I think they're sandbagging. Plus Murphy is supposedly just as good as Cedric. So we'll find out."

(Additional reporting by Patricia Zengerle; Writing by Kevin Drawbaugh; Editing by Jason Szep and Jonathan Oatis)