simg,
I disagree.
Pre-election pledges could have conditional clauses and exceptions to
make it clear in what circumstances they would apply, and when they
could not. It is pointless to make an unconditional pledge and then
break it because circumstances have changed. What was the point then
of making the pledge in the first place? It seems very much like it
was only to gain votes. (And some people do base their voting
decisions on the party manifesto.)
If pre-election pledges were to be made legally binding on those who
are elected, they would be much more careful about the promises that
they made in the election campaign. And this could only apply to
pledges made in the knowledge that they would be legally binding; it
would not be desirable to make something legally binding that had been
said before this mechanism was put into place.
Business contracts have terms and conditions that are expected to hold
in varying circumstances. Why shouldn't there be "contracts" in place
for the MPs that we vote for to ensure that they stick to their word
and distinguish between intentions and pledges?