Or else for what wasn't a safe seat to become one once the PM represents it.
I'm reminded of the Crazy Land of Canada, where sitting PM Mackenzie King lost his seat for the Ontario riding of York North in 1925. Still, being the leader of the Liberals, he needn't worry, Charles McDonald who enjoyed a 28.7% majority in the riding of Prince Albert in Saskatchewan duly resigned to allow Mackenzie King to get back into the House of Commons. He won no less than 77.5% in the resulting by-election.
In the federal election that followed in 1926, that dropped to 64.9%.
In the federal election of 1930 it was down to 53.4%.
In 1935, as the Liberals' support increased dramatically across Canada due to anger with how the Conservatives were handling the Depression and the Liberals won a landslide, Mackenzie King's voteshare in his own riding only increased from 53.4% to 54.7%.
In 1940, as the Liberals obtained nationally what was then their best result ever, in Prince Albert, the Liberal vote actually fell further to 46.0%.
And in 1945, as Mackenzie King was emerging from World War 2 as the leader of one of the victorious nations... The Liberal vote in Prince Albert fell to 40.3%, and Mackenzie King actually lost the seat to the CCF.
Mackenzie King managed the remarkable feat of getting a safe seat, and somehow during his time as Prime Minister was able to wear it down to a marginal. In fact, since 1953, no Liberal in Prince Albert has managed better than to come in
third. (Well, except for the year 2000.)