If Debbie Dingell succeeds her husband John in Michigan's 12th Congressional district - as seems likely - it will mark a slightly odd milestone: She'll be the first woman to take over her husband's seat while her husband is still alive.

Plenty of women have inherited their husband's seats before - 47 to be exact, 8 in the Senate and 39 in the House. But according to Debbie Walsh of the Center for American Women and Politics at Rutgers University, in all those instances the woman took over the seat after her husband had passed away. The mechanics are slightly different for the House and Senate - empty House seats are filled via a special election, while Senate seats are typically filled via appointment. "Widow's succession" or the "widow's mandate" is the technical term for when an empty seat is filled by the spouse of the deceased legislator.

"Widow's succession used to be THE way that women got into Congress, with very few exceptions," explains Walsh. The practice peaked in the mid-twentieth century. "There was a period when you could look at all the women serving in Congress, and a majority had initially gotten in that way." It's declined since, but still persists - Lois Capps (D-CA) and Doris Matsui (D-CA) are the only two women currently in Congress who initially inherited their husbands' seats.

The idea behind the practice was continuity, the notion that the women would complete the work their husbands started. "For the parties, these women were placeholders," Walsh says. "The idea was to get somebody in and then regroup, and keep intraparty fights from happening."

I was curious whether it worked out this way in practice: Did the widows govern similarly to their husbands while in office? One way to get at this question is to look at the comparative ideological leanings of both spouses while in office. For each husband-wife pair, the chart below plots ideology scores developed by political scientists Keith Poole and Howard Rosenthal - positive for conservative ideology, negative for liberal. Because these scores are derived from legislators' voting records, they serve as a useful proxy for overall governing behavior. Use the buttons to sort the chart by husband or wife's political ideology, the difference between the two, or the date the wife took office.

In most cases wives do in fact govern similarly to their husbands - on average wives' ideological scores are slightly more liberal than their spouses, but not by much. California's Mary Bono is notable among recent widows for being significantly more conservative than her husband Sonny. Interestingly, the greatest spousal ideological differences occurred in the early part of the 20th century - of the five largest ideology score differences, four were from women who took office in the 1920s and 1930s.

Moreover, in the 1920s and 1930s women tended to govern closer to the center than their husbands - Democratic women were more conservative than their husbands, while Republican women were more liberal. But this trend fades after that point. Conversely, since the 1970s most of the widows have generally governed similarly to their husbands.

Which brings us back to Deborah Dingell. If the recent trends hold true, it's safe to assume she won't make any radical departures from her husband's record. On the other hand, by taking office while her husband is still alive she's in somewhat uncharted territory.

One final thought: I asked Debbie Walsh of the Center for American Women and Politics how many instances there have been of men stepping in after their wives pass away - widower's succession, if you will. She told me there had been exactly zero.