Sen. John Walsh has admitted to plagiarizing portions of his master’s thesis. Although a universe exists in which the Democrat could still beat Republican Rep. Steve Daines in Montana’s 2014 Senate race, it’s not a universe near to ours; Walsh had little chance before the scandal and is all but finished now.

Before the plagiarism news broke, Walsh hadn’t led in a single poll this year. Some recent polling had indicated he was closing the gap, but my analysis of polling from January to June gave him only a 3 percent chance of winning. Our semiofficial FiveThirtyEight odds, based on polling, candidate quality, fundraising and other factors, gave him only a 15 percent chance of winning in November.

In addition, President Obama’s approval rating in Montana is around 35 percent. Combining Walsh’s deficit and the president’s low standing, it would have been difficult for the senator to make a comeback.

The scandal makes that hill even harder to climb. According to a 2011 study of U.S. senators by Nicholas Chad Long, an assistant professor at St. Edward’s University in Texas, objectionable behavior like Walsh’s typically costs a candidate about 10 percentage points in his re-election campaign. In other words, Walsh’s current 10-point deficit would, on average, double to nearly 20 points.

The only Senate candidate to come back from something close to a 20 percentage point deficit at this point in a general election campaign over the past 10 years was Virginia’s Jim Webb in 2006. Webb, of course, was the beneficiary of another scandal — then-Sen. George Allen’s “macaca” comment.

We’ll have to wait to see whether Walsh can defy precedent, but Republicans who were counting on flipping this seat as one of the six they need to regain control of the Senate can now count on it that much more.