Longfield ran as the
Liberal candidate for the
riding of
Guelph in the
2015 federal election. The riding was previously held by Liberal
member of Parliament (MP)
Frank Valeriote who had opted not to run for re-election. Longfield was successful in retaining the Guelph seat for the Liberals by a wide margin, earning nearly 50 per cent of the popular vote and finishing more than 15,000 votes ahead of
Conservative candidate Gloria Kovach. After being elected, Longfield promised to vote to increase funding to the
CBC and to stop the phase-out of door-to-door mail delivery. The latter was a part of the Liberal platform as described by Justin Trudeau in a September 25, 2015 letter: "we will also stop the Harper Conservatives' plan to end door-to-door mail delivery and ensure Canadians receive the postal service on which they rely." Longfield was re-elected in the
2019 and
2021 federal elections. On June 28, 2023, he announced that he would finish his term, but would not run in the
45th Canadian federal election. ==Personal life==