If the Speaker of the House of Commons is impartial then why are Labour and the Lib Dems trying to ensure a Conservative does not become the next one?

There are increasing numbers of reports that Labour are trying to ensure that the next Speaker of the House of Commons is not a Conservative. The plan seems to be to get Sir Menzies Campbell elected before the next election rather than risking a Conservative majority electing a Conservative Speaker after the election. Apart from the fact that the convention of alternate Labour and Conservative Speakers was broken by Labour after the 1997 general election when they were in full "political wing" of the British people mode, it does strike me as odd that there is so much manoeuvring about the election of an "impartial" official.

Is this Labour party scheming recognition that Michael Martin has not been impartial and that Labour want to ensure that any incoming Conservative government will have none of the advantages of a biased Speaker that they have enjoyed since 1997?