By-elections are one-off elections in seats where, for example, the sitting MP has stood down or died.
"The jury is out on whether the Trump election was the beginning of a new movement or whether it was a one-off election," Stewart told VICE News in an interview.
"Anticipating what the turnout is going to be in such a one-off election is extremely difficult, and that led to different assumptions by different pollsters," Podhorzer explained in a conference call with reporters on Wednesday.
There is a need for the Lib Dems, Greens and Labour in the constituency to get together to see if an accommodation can be reached with one joint candidate as a one off – possibly via a primary election.
"No more off elections," shouted Mr Barber.
You can't write this one off.
"One" is the the number of years that have passed since the last presidential election, which technically makes 2013 an "off" election year.
Landrieu has fought off election scares before.
He returned himself to office in June after a violent one-man run-off election.
In addition, unions draw on their political funds to provide one-off donations, mainly at election times, amounting to about £12m.
Debates have been held in Australia since 1984; in Canada regularly since 1979 after a one-off in 1968 and election debates are staged in most European countries.
