Labor's Ian Gibson resigned his seat in the House of Commons, after being implicated in the expense reimbursement scandal that I wrote about, here. The Conservative Party won the by-election (what Americans would call a "special election") to replace Gibson.
As Irving Mills wrote, to music composed by Duke Ellington, "it don't mean a thing if it ain't got that swing." Well, this by-election had that swing.
Gibson won the seat for Labor at the 2005 general election, with a majority of 5,459. Chloe Smith, the victorious Conservative candidate in yesterday's by-election, turned that around to a majority of 7,348 for her party. (It seems to me that should be called a "plurality" rather than a "majority", but, oh well, let's call the whole thing off.) In percentage terms, that amounts to a swing of 16.5% away from Labor, to the Conservative Party.
That swing percentage is watched closely, in terms of what it might indicate for the general election that must be held by next June. There's an analysis that oversimplifies the situation, but which has some validity. It says, if the Conservatives make a 16.5% gain in every constituency across the U.K., how many additional seats will they win?
The BBC quotes a not-unbiased observer, Theresa May, a member of the Conservative shadow cabinet:
We've overturned a Labour majority of 5,500 to a Conservative majority of over 7,000. If we hold this seat at the general election, we'll have a majority of over 100.
If that prediction holds up, it will be the biggest Conservative victory since Margaret Thatcher won a third term at the 1987 general election.