Sen. John Walsh (D-Mont.) probably isn't returning to Congress in 2015. That was true this morning, and it's even more true now that we know he apparently plagiarized a significant portion of his master's thesis less than a decade ago.

The New York Times's Jonathan Martin on Wednesday afternoon revealed what appears to be some pretty extensive plagiarism in the senator's thesis at the Army War College in 2007. To wit:

An examination of the final paper required for Mr. Walsh’s master’s degree from the United States Army War College indicates the senator appropriated at least a quarter of his thesis on American Middle East policy from other authors’ works, with no attribution.
Mr. Walsh completed the paper, what the War College calls a “strategy research project,” to earn his degree in 2007, when he was 46. The sources of the material he presents as his own include academic papers, policy journal essays and books that are almost all available online.
Most strikingly, each of 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,” is taken nearly word-for-word without attribution from a Carnegie Endowment for International Peace document on the same topic.

Walsh hasn't quite admitted to the plagiarism, but his campaign isn't disputing the story and attributes the mistakes, in part, to a difficult time in his life, after someone he served with in Iraq committed suicide.

While the story immediately dominated what has been a massive political news vacuum, it's also worth noting that this kind of thing is hardly the career-ender it once was. While politicians in the past, including Vice President Biden, have seen their careers suffer significantly from plagiarism, more recent history suggests academic misconduct isn't anything close to a deal-breaker for the American voter.

Those who appear to have plagiarized in some form in recent years include no less than President Obama (who appeared to borrow some lines from Massachusetts Gov. Deval Patrick on the 2008 campaign trail), Canadian Prime Minister Stephen Harper, former senator Scott Brown (R-Mass.) and, most recently, Sen. Rand Paul (R-Ky.).

And it's not just plagiarism. As Nathan Gonzales noted earlier this month, the Senate is packed with politicians with similar foibles on their record -- whether it's plagiarism, lies on their resume or questions about their roles in some unsavory events.

Here's a sample:

In 2010, The New York Times pointed out inconsistencies between Democratic state Attorney General Richard Blumenthal’s rhetoric and his military service during the Vietnam era. It became a major issue in the campaign, but Blumenthal prevailed, 55 percent to 43 percent, over former wrestling executive Linda McMahon.
Two years later, Democratic Rep. Christopher S. Murphy ran for the Nutmeg State’s other Senate seat. In September of the election year, it came to light that the congressman missed multiple mortgage payments and the bank had started the foreclosure process on his home. Murphy defeated McMahon, 55 percent to 43 percent, that November.
The Connecticut senators are certainly not alone in having facing potentially disastrous headlines during the campaign.
In 2008, comedian and former radio talk show host Al Franken had to answer questions about his involvement in a never-aired “Saturday Night Live” sketch that involved “60 Minutes” correspondent Lesley Stahl being raped. And he paid $70,000 in back taxes in 17 states during the race. Franken defeated then-Sen. Norm Coleman, R-Minn., with 42 percent and a margin of 312 votes.
In 2012, Democrat Elizabeth Warren faced multiple days of questions about her family’s Native American heritage and whether she used it inappropriately to get a teaching position at Harvard Law School. She defeated Sen. Scott P Brown, R-Mass., 54 percent to 46 percent.
Democratic candidates are not the only ones hit some ruts on the campaign trail.
In 2010, Republican Mark S. Kirk falsely claimed to have received an Intelligence Office of the Year award from the U.S. Navy. The congressman went on to defeat state Treasurer Alexi Giannoulias, the Democratic nominee, 48 percent to 46 percent.

We think this point is well-taken, and we all have a tendency to over-estimate the importance of something like plagiarism the moment it happens. Walsh could very well rebound just fine from this.

But it certainly hurts him. For a few reasons:

1) He was already down. A major difference between the names above and Walsh is that he's already fighting an uphill battle, with polls showing him trailing Rep. Steve Daines (R) by between seven and 18 points. It's one thing to have a story like this come out when you have some margin for error or you're tied. Walsh was already fighting a losing battle.

(We would also note that some of the names above, including Blumenthal and Murphy, won by smaller margins than they probably should have.)

2) Walsh isn't a known quantity. All of the people above have pretty lengthy political careers to speak of, and even Paul has become well-known in the time since he was first elected in 2010. When that's the case, it's easier for people to give you the benefit of the doubt. Walsh, though, is a political newcomer who has never won office in his own right and was thrust into the big-time relatively recently. Are Montana voters going to stand by a guy who they've never really even stood by in the first place?

3) This has to do with credentials. Of all the above, the one that probably hurt the most was Blumenthal's -- mostly because he appeared to inflate his resume and it had to do with military service (in Vietnam). Walsh's misdeed isn't on the same level, but it's still cheating to advance your career. As Martin reports, his master's degree was part of the reason he was named adjutant general of the Montana National Guard in 2008.

4) It fits a pattern. The biggest GOP attack on Walsh before Wednesday was that, in 2010, he was reprimanded for soliciting National Guard troops to join a private association -- the National Guard Association of the United States -- for which he was seeking a leadership post. Such pressuring is a no-no. It's pretty easy to see GOP attack ads combining these two items in his past to make Walsh look like someone who will do whatever it takes to get ahead.

The four arguments above notwithstanding, we think plagiarism ranks pretty low on the list of things that voters care about. And it certainly helps his credibility that Walsh is no less than a bronze-star recipient -- something his allies were quick to point out Wednesday afternoon in his defense.

But to the extent that Walsh was already an underdog and that this causes people who don't really know him to question his character, it's a pretty bad day for his campaign.