The Labour deputy leader will claim that the move by Tory MPs to safer seats, or out of politics altogether, is evidence that they no longer believe they can hold on to marginal seats in the general election.
Mr Mawhinney, the Tory MP for Peterborough since 1979, is considering a move to the new, safer seat of North West Cambridgeshire.
He is one of a number of Tory MPs who are considering moves to other constituencies, following boundary changes which are likely to reduce their majorities.
Other Tories on the move include David Amess, the Tory MP for Basildon, who has been adopted for Southend West; Peter Bottomley (Eltham); Nick Hawkins (Blackpool South); Nicholas Soames, who is quitting Crawley for Mid-Sussex; and Sir George Young, the Secretary of State for Transport, who may be on his bike from Ealing Acton to Maidenhead.
The merry-go-round of Tory MPs was intensified by Dudley Fishburn, the MP for Kensington, who announced that he was giving up his marginal Tory seat with a majority of 3,548 to retire from the House of Commons.
Mr Fishburn, 48, a former editor of the Economist, incensed colleagues by saying that there were too many MPs and not enough for them to do.
He said: "Six-hundred-and- fifty-nine MPs cannot (short of being outrageous busybodies) find enough to do in our small, already well-governed kingdom. Good heavens, we ran an empire on a great deal fewer."
Mr Fishburn's decision to retire at the election follows that of George Walden, the Tory MP for Buckingham, who is giving up because he is unable to continue "sweet-talking the public like infants".
Meanwhile, Tim Collins, the Prime Minister's "spin doctor" at the last general election, yesterday secured a safe seat at the first attempt.
Mr Collins, 31, former head of communications at Tory Central Office, now number two in the Prime Minister's Downing Street policy unit, was adopted for Westmorland and Lonsdale, where Michael Jopling, a former minister, had a majority of 16,436 at the last election.
Mr Collins said: "The fact I made clear I am a John Major loyalist played in my favour. It is further evidence of the strength of support for the Prime Minister at the grass roots after the leadership election."Reuse content