The tendency to tactical voting benefits established parties disproportionately because a vote for a libertarian, say, might “split” the anti-Liberal/Conservative vote etc. and let the wrong blighters in.

Here is a voting reform proposal which might be sold to the incumbents before they realise the consequences. In addition to a positive vote, each voter would have a veto. The vetoes would be counted first, and any candidate with more than a threshold of vetoes would be knocked out of the positive vote count. The positive votes would would have to be numbered preferences so that if a voter’s preferred candidate were eliminated on the veto, the voter still gets to decide second best.

The benefits of the veto are especially apparent when there are more than two candidates for a seat. Under “first past the post” or STV tactical voting can can cause a candidate, detested by the majority of the voters to be elected. The veto stage would eliminate the most detested candidates, and therefore improve acceptability of the result.

One could use the veto to provide a meaningful “none of the above” option.

I Imagine that the incumbent parties, who are expert at negative campaigning would intensify the negativity in the hopes of knocking out their main rivals. And no need to make promises they can’t keep.

If the veto quota proportion were set at /(n+1) where there are n candidates, then with luck the incumbent tribes would knock each other out, while fresh parties would get a fair shot.