Sen. Ben Nelson, D-Neb., sporting a Mizzou logo courtesy of Sen. Claire McCaskill

WASHINGTON - The morning of tonight’s showdown between the No. 21 Nebraska and No. 24 Missouri college football teams, Sen. Ben Nelson, D-Neb., learned that turnabout is fair play when dealing with Sen. Claire McCaskill, D-Mo.

Escalating a back-and-forth prank war, McCaskill and a team of staff members infiltrated Nelson’s office Wednesday night and posted a half-dozen doctored photos featuring Nelson in compromising garb - a Mizzou helmet, a black-and-yellow tie and even a Tiger logo painted on his smiling face.

Nelson, who proudly displays a Cornhusker flag seen from his office foyer, retaliated this morning by clipping a Nebraska state flag over McCaskill’s Missouri flag outside her front doors. The Nebraska flag, of course, was taken down by this afternoon.

The prank is just the latest in a playful battle between McCaskill and Nelson. In 2007, when McCaskill was about to be sworn in to the Senate, Nelson rehashed an old joke by asking McCaskill which Bible verse she planned to recite during the ceremony (there is no Biblical recitation during the ceremony). Nelson let McCaskill nervously flip through her Bible for a verse before whispering, “I love practical jokes.”

And last year, near the eve of the Nebraska-Missouri football game, Nelson sneaked into McCaskill’s office while she was on the Senate floor, rearranged furniture, hung a Nebraska flag and posted photos of himself in full hunting gear on her walls.

McCaskill raised the bar this year by posting a doctored photo of Nelson on Twitter bearing the Mizzou Tigers logo on his cheek, adding, “Great picture. Sen. Nelson’s new official portrait. Don’t miss this.”

In both office pranks, the senators have fallen victim to mischievous staffers, who were complicit in the pranks by letting McCaskill and Nelson into each other’s offices.

“They have a very fun, light-hearted back-and-forth when it comes to Missouri and Nebraska sports,” McCaskill spokeswoman Maria Speiser said.