The last key job of an MP is to debate the policies being put forward by the government of the day. They are given the chance to this in Commons debates held either by the government themselves or the opposition. This is often a more cooperative activity, but if there are serious agreements on a policy area between the opposition and the government, these debates can become quite heated, especially if they are high profile debates on matters such as Brexit, the NHS or major government cuts or tax changes.
MPs also have the chance to hold debates on issues which particularly concern them through the Backbench Business Committee. The Backbench Business Committee is made up of other MPs (like select committees) and they take requests from MPs who have to persuade the committee to give them a debating slot a matter they wish to have a discussion on. They number of slots they are able to allocate is decided by the government.
Some MPs in both the governing party and the opposition party become whips. The Whips’ main job is to make sure that the MPs in their party vote with the party line on a motion or a bill. As the UK Parliament website states:
“Every week, whips send out a circular (called 'The Whip') to their MPs or Lords detailing upcoming parliamentary business. Special attention is paid to divisions (where members vote on debates), which are ranked in order of importance by the number of times they are underlined.”
“Important divisions are underlined three times - a 'three-line whip' - and normally apply to major events like the second readings of significant Bills.
“Defying a three-line whip is very serious, and has occasionally resulted in the whip being withdrawn from an MP or Lord. This means that the Member is effectively expelled from their party (but keeps their seat) and must sit as an independent until the whip is restored.”
Whips can put various forms of pressure on MPs, including the withdrawal of the whip, to make members vote with their party even if they don’t agree with their party on the matter. MPs know that if they do not consistently vote along party lines they will never be promoted to a higher position within the House of Commons where they might have more influence to make the national changes they want to make.
The power of the Whips is limited, however. Some Members of Parliament, either because they are nearing retirement or because they have no interest in being promoted (or because they can’t bring themselves to vote for something they believe to be wrong), will ignore the whips no matter what they say and vote the other way if that is how they wish to vote.
Also, as we have seen in the case of Jeremy Corbyn’s leadership of the Labour party, if MPs have no faith in the ability of their leader to win an election, Whips have little power to enforce even a three-line whip on votes when a significant number of MPs strongly wish to vote the other way.