[1] McCallum has been on the executive of the Bluegreens (the National Party environmental issues advisory group) since 2001, spent ten years serving on the National Party board (2005–2015), and was vice-president of the Northland branch of Federated Farmers.

[1][4] McCallum contested the selection for the National Party candidacy for the 2015 Northland by-election, but lost out to Mark Osborne.

[7] McCallum was selected by the National Party to contest the Northland electorate at the 2023 election.

[9] McCallum identified the cost of living and regulatory burden on farmers as issues in his electorate.

[1] After the election, he said "It’s a huge responsibility and Northland has quite a lot of challenges and big issues that we can’t fix overnight.