Bruce Hyer's comments on the difficulties he faced when he wished to vote against the NDP's position on the long-gun registry highlights the difficulties and pressures felt by MPs when they vote against their party's wishes. It was quite interesting to see that when a vote is in question, even the so-called "progressive" NDP are no different than the Liberals or Conservatives when it comes to enforcing party discipline. In all three parties it's the leader's way or the highway.
Party discipline is a fact of life in federal Canadian politics. It most often shows up on the floor of the House of Commons where MPs dutifully stand and vote the party line on any number of issues or motions. The public would be surprised at how often MPs are handed a sheet of paper that tells them how they are to vote on various motions, proposed amendments to legislation etc. On the government side of the House the "Vote Rationale" is provided to Conservative MPs prior to every vote.
A vote rationale is even provided to Conservative MPs prior to a vote on a Private Member's Bill. However, there remains some flexibility for the MPs unless the bill impacts the party platform or public positions taken by the leader. In that case any MP not toeing the line can expect considerable pressure to be exerted.
On every vote, the Whip will note how his/her MPs voted, who was present, who failed to show (who had urgent and pressing business in the riding that day) and every MP knows that going against the party line may have repercussions. These can extend from being ignored by colleagues, to eventual reassignment to some of the least desired committees, to lack of speaking time, or few if any questions if the MP is an opposition member. There are an infinite number of ways that the leader's office can remind the MP of their displeasure.
This discipline or "thought control" extends beyond the Commons to committees as well. Here members are often briefed by staff from the various leaders' office, minister's office, or critics' office as to how they should vote. By necessity on the government side discipline is usually tighter as its government legislation or positions that are under attack. Disagree with the party position and you will quickly find another MP substituted into your slot on the committee when a key vote comes up. You might even find yourself removed from that committee.
The ultimate power that rests with all leaders is their ability to have an MP thrown out of caucus. Even if that is not the case, every candidate whether brand new or sitting MP, knows that the leader must sign their papers at election time. It is a brave MP who stands on principle and refuses to knuckle under.
This is hardly serving the best interests of your constituents, and parties over the years have promised more freedom for MPs, more free votes etc., but little comes of it. All too often deviance from the party line by an MP becomes a media story and it plays as an embarrassment of the respective leader. It is no wonder then that party leaders react so strongly when this happens.
All of this helps to contribute to the negative perception that the general public has of parliament and MPs. It will only get worse as hyper-partisanship is now deeply entrenched in the House and all parties regard the other sides' MPs as their enemy and not their colleagues. Voting with the enemy is not something most MPs will ever stand to do.