In 1922, Campbell defeated several other contenders to become the United Farmers of Manitoba (UFM) candidate in Lakeside, north of Winnipeg.

The UFM swept Manitoba's countryside in 1922, and Campbell defeated his only opponent, Conservative Herbert Muir,[1] by about 500 votes.

[1] In this capacity, he was responsible for overseeing a rural electrification program, which created the framework for Manitoba Hydro.

Bracken's government was expanded into an all-party coalition in 1940, with the Conservatives, Manitoba Co-operative Commonwealth Federation and Social Credit all holding cabinet positions.

On the latter occasion, Campbell defeated Conservative leader Errick Willis in a vote of the coalition caucuses to become the province's next Premier.

The Manitoba Tories had more or less vanished at the grassroots level during the coalition government, and in the 1953 election, Campbell's Liberal-Progressives won a convincing victory (32 of 57 seats).

Campbell often had poor relations with the federal Liberals in the late 1950s, despite the fact that Garson was a powerful cabinet minister for the party.

John Diefenbaker's upset victory of 1957 was partly based on unexpected support from Manitoba—Campbell later claimed that he contributed to the federal Liberal defeat.

The provincial election of 1958 resulted in a hung parliament, with the Tories winning 26 seats against 19 for the Liberal-Progressives, and 11 for the CCF, and one Independent.

The next year, Roblin's Tories won a parliamentary majority and Campbell's Liberal-Progressives were reduced to 11 seats.

He continued to serve as MLA for Lakeside until standing down in 1969, and exercised a powerful influence over the Liberals during this time.