On the understanding that they'd work towards implementing the policies, platform and ideology they stood on - not just palm it off to some unknown who doesn't in the least share those ideals.
If the voters consider their MPs have betrayed their promises, they ought not to reelect them during the next parliamentary elections. This is democracy.
I'd make certain to do that, but it seems like when only the Lega Nord are actually not partaking in this betrayal, it's not that easy.
Put simply, if you voted for, say, PD and upon winning they and their coalition installed a cable of Berlusconi loyalists to power for five years, you'd have no problem labelling that undemocratic.