In 1981, his friend Michel Gratton convinced him to be a Liberal candidate in Pontiac, a newly reformed riding that now included the territory of Aylmer.
[1] Being born of an anglophone father and a French-speaking mother, he had the advantage of being equally at ease in both communities.
On his first attempt, in 1981, he was elected easily; however, the Liberal Party under leader Claude Ryan formed the opposition.
Middlemiss was re-elected four times and never faced a serious challenge in the heavily federalist riding, only once dipping below 50% in 1989 due to the English-speaking minority rights Equality Party challenging the Liberal Government over the invocation of the notwithstanding clause of the Canadian Charter of Rights and Freedoms to override a Supreme Court of Canada ruling overturning parts of the Charter of the French Language.
In 1985, the Liberals regained power with Robert Bourassa at the head of the party.