Westminster by-elections are normally contested by all the main parties but have a different character to general elections:-
By-elections tend to be a judgement on the performance of whichever party is in Government. Governments are often unpopular in the middle of their term in office because they have had to take difficult economic decisions and voters use a by-election to protest against this or against other policies of the Government.
This benefits the main opposition party or perhaps a smaller party if they are strong in that constituency. By-elections such as that in Clacton, demonstrated a general reaction against the main political parties to the benefit of UKIP.
In a general election, most people are instead intent on choosing a Government as so tend to see the choice as one between the two main parties rather than simply focussing on Government performance.
In a by-election the spotlight is on the local candidates far more than in a general election and so whether they have local ties and their personality may be a bigger influence on voters.
In the Bermondsey by-election of 1982 Peter Tatchell, a gay rights campaigner, lost a safe Labour seat because of a homophobic campaign by the tabloids as well as locally.
In contrast, Shirley Williams had been a very well-known and liked Labour politician but had left to join the Social Democratic Party and her personality and reputation enabled her to win the safe Conservative seat of Crosby in the 1981 by-election.
If the constituency is expected to be closely connected then national and local media will give much more publicity to the candidates.
Particular local issues may have considerable prominence in a by-election such as the grooming of young girls by gangs and the failure of the authorities to act in the Heywood and Middleton by-election in 2014 when Labour almost lost a previously safe seat.
In a general election the focus is far more on a choice between party leaders and national party image and the key party policies and so the nature of local candidates, while still a factor, becomes less significant. National issues such as the economy and the NHS are likely to predominate.
In a by-election smaller parties are able to throw all their resources into the campaign if it is a constituency where they feel they have a chance of winning. The Liberal Democrats have been particularly good at by-election strategy over the years with gains from the two main parties.
In a general election, the Conservative and Labour parties have more party activists and more money for campaign advertising, targeted letters and so on whereas the smaller parties are spread more thinly and have to carefully select which constituencies to concentrate on.
Turnout in by-elections is normally lower.
The main parties may have difficulty in getting all their traditional voters out whereas third parties may be able to generate enthusiasm among their supporters and fewer votes are needed to win the seat.
Turnout in a general election may be nearly twice as much as in a by-election and so the result is more likely to be typical of the long-term trends in that constituency.
