WASHINGTON (CNN) - Late Wednesday night, Sen. Ben Nelson's office was littered with doctored, deliberately embarrassing photos of the Nebraska senator. There's no investigation looming - Nelson already knows the culprit: a fellow Senate Democrat, who boasted of her exploits on Twitter.
Ahead of the Nebraska-Missouri football game, Missouri Sen. McCaskill and her press secretary were granted access to Nebraska Sen. Nelson's office for the latest round in a prank war that stretches back years.
The senator and her aide replaced official and family portraits.with various doctored pictures of Nelson sporting Missouri's black and gold, and posing with the Mizzou tiger mascot, according to Nelson spokesman Jake Thompson. "Important timely new official photo. Glad @SenBenNelson has finally come around," McCaskill tweeted, along with a picture of Nelson sporting the Mizzou logo on his face, a gold Missouri lapel pin and a gold and black tie.
Both teams have 4-1 overall records, but Nebraska is atop the Big 12 Conference's North Division with a 1-0 conference record while Missouri is 0-1. The latest college football poll ranks Nebraska No. 21, while Missouri is 24th.
Last spring, while McCaskill was presiding over the Senate floor, Nelson was granted access to her office. He went in and rearranged some of her furniture, hung a big Nebraska flag and placed various pictures of himself throughout the office, McCaskill spokeswoman Maria Speiser recounted to CNN.
The pranks started in 2007 the day McCaskill was to be sworn in into The United States Senate for her first term when Sen. Nelson asked McCaskill which biblical verse she had chosen to recite for her ceremony.
"She went into a quiet panic," Speiser recalled. "She started to page through the Bible, thinking frantically 'what am I going to do?'"
"[Nelson] let her go on for a couple minutes thinking she was in trouble for having forgotten she needed a Bible verse, before he whispered to her 'I just love practical jokes.'" Speiser said. Nelson played the same prank on Hillary Clinton before she was sworn in in 2001.
What was Sen. Nelson reaction when the tables were turned?
"He thought it was really funny," said Thompson, who has twice been a target of the senator's pranks.
While the pranks usually happen about once a year, Speiser said she wouldn't be surprised if Nelson tries to retaliate ahead of schedule - "but who knows when that will be," she said.