What is a marginal seat?
A marginal seat is a Parliamentary constituency where the gap between the winning candidate and the nearest challenger is relatively small.
That means a comparatively small change in votes could cause the seat to change hands at the next election.
In short
A marginal seat is competitive.
A safe seat has a much larger winning margin.
There is no single legal cut-off that turns a constituency into a marginal — it is a descriptive political term.
How is a constituency majority calculated?
The simplest majority is:
winner's votes − runner-up's votes
For example:
| Candidate | Votes |
|---|---|
| A | 20,500 |
| B | 19,800 |
| C | 5,000 |
Candidate A's majority over Candidate B is:
700 votes
That would normally be regarded as a very marginal result.
What is a percentage majority?
Analysts also compare the winning margin with the total number of votes cast.
That helps compare constituencies of different sizes.
For example, a 2,000-vote majority might be fairly safe in a seat with very low turnout but much more marginal in a constituency where many more people vote.
The House of Commons Library often analyses marginality using the winning margin as a percentage of votes cast.
Is there an official definition?
No.
There is no law stating that "under 5% = marginal".
Analysts often use bands such as:
- under 2%;
- under 5%;
- under 10%.
But the choice depends on the purpose of the analysis.
The important idea is simply that the smaller the winner's lead, the fewer votes need to change for another candidate to win.
How many marginal seats were there in 2024?
House of Commons Library analysis found that the 2024 general election produced a large increase in marginal seats.
It identified:
- 115 constituencies won by a margin of 5% or less of total votes cast;
- 46 constituencies won by a margin below 2%.
That meant nearly one in five seats had a winning margin of 5% or less.
These figures describe the 2024 result. The marginal map can look very different at a future election.
Why do parties target marginal seats?
Campaign resources are limited.
Parties have to decide where to deploy:
- candidate and leader visits;
- volunteers;
- leaflets;
- digital advertising;
- canvassing;
- campaign spending;
- organisational effort.
Under FPTP, gaining another 5,000 votes in a seat a party already wins by 20,000 does not create another MP.
Moving enough voters in a marginal seat can.
This gives parties a strong incentive to concentrate resources where the constituency result is most likely to change.
Does a "safe seat" never change hands?
No.
Safe describes the previous electoral position, not a guarantee.
A major national swing, local issue, candidate change or political realignment can cause apparently safe constituencies to change party.
Marginality is therefore based on evidence from a particular election and assumptions about future competition.
Are all marginals two-party contests?
No.
Some constituencies have a clear first-versus-second contest.
Others are competitive between three or more parties.
This matters when thinking about swing and tactical voting.
A simple two-party swing calculation, for example, is much less informative in a seat where several parties are highly competitive.
In Road to Number 10
The simulation tracks swing seats because marginal constituencies are central to campaign strategy under FPTP.
A successful campaign manager has to decide whether to:
- defend vulnerable seats;
- target potential gains;
- build national vote share;
- concentrate resources in specific battlegrounds.
This creates the strategic tension at the heart of the game.