An ordained minister, Calvert served as minister of Zion United Church in Moose Jaw from 1979 to 1986. He then entered provincial politics, running as a New Democrat, and was elected MLA for Moose Jaw South. He was re-elected in 1991 and 1995 in the riding of Moose Jaw Wakamow, and in 2001 in Saskatoon Riversdale. He became premier on February 8, 2001, after winning the NDP leadership campaign to replace outgoing leader and premier Roy Romanow. Calvert and the NDP edged out the right-wing opposition Saskatchewan Party in a general provincial election (Saskatchewan general election, 2003).