Sen. John Walsh (D-Mont.), right, and his son Michael on Capitol Hill in February. Walsh's thesis written for the U.S. Army War College contains unattributed passages that appear to be taken from previously published papers. (Lauren Victoria Burke/Associated PRess)

The two hardest words for a politician to say: “I’m sorry.”

Sen. John Walsh (D-Mont.) provided fresh evidence of that fact this past week after a New York Times report that he had plagiarized large portions of a research paper he wrote while a student at the U.S. Army War College.

Wrote Jonathan Martin, who broke the news:

“Most strikingly, the six recommendations Mr. Walsh laid out at the conclusion of his 14-page paper, titled ‘The Case for Democracy as a Long Term National Strategy,’ are taken nearly word-for-word without attribution from a Carnegie Endowment for International Peace document on the same topic.”

Pretty clear-cut, no?

What you might have expected Walsh to do was simply apologize. After all, plagiarism typically amounts these days to a venial sin in politics; Sen. Rand Paul (R-Ky.) survived plagiarism accusations recently, while Sen. Scott Brown (R-Mass.) weathered them in 2011.

But that’s not what Walsh, who is trailing his GOP opponent in polls leading up to November’s midterm election, decided to do. Rather than just apologize and move on, his campaign chose to savage the story as the work of Republican opposition researchers — um, would that make it not true? — and give a number of excuses for why Walsh did what he did.

In an interview with the Associated Press on Wednesday, Walsh sort of blamed post-traumatic stress disorder from his service in Iraq. “My head was not in a place very conducive to a classroom and an academic environment,” he said. Which, of course, raises the question: Why didn’t he simply ask for a leave rather than plagiarize? And in a Thursday statement, the campaign called the issue an “unintentional mistake.”

John Walsh, for making excuses when you should have been apologizing, you had the worst week in Washington. Congrats, or something.

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Each week, Chris Cillizza awards the worst week in Washington to an inhabitant of Planet Beltway who stands out for all the wrong reasons. You can check out previous winners or e-mail Cillizza with candidates. You can also read more from Outlook and follow our updates on Facebook and Twitter.

Chris Cillizza writes “The Fix,” a politics blog for the Washington Post. He also covers the White House.