James Stevens has resigned as chief of staff to the South Australian premier, Steven Marshall, in a bid to replace Christopher Pyne in the federal seat of Sturt.

When the defence minister announced on Saturday he would not recontest the Adelaide seat he has held for 26 years, he told reporters Stevens “fits the criteria” to be his successor.

On Sunday Stevens tendered his resignation to Marshall “with immediate effect” so he can contest preselection to be the Liberal candidate for Sturt at the May federal election.

Stevens has been Marshall’s chief of staff for more than six years, saying in his resignation letter that the “undoubted highlight was of course last year’s historic election victory”.

Despite pressure for the Liberal party to increase its female representation in Canberra, Pyne said his replacement should be selected “entirely on merit”.

He said while a decision on his replacement would be up to electors, “James Stevens is a very fine individual” with the credentials for the job.

Stevens is also a federal electorate conference president and has been Sturt campaign manager for the last four elections.

In his resignation letter, Stevens said it would be negligent of him and against the interests of Marshall’s government for him to continue his role, having decided to contest preselection.

“This role must always be held by a person exclusively focused on serving you, and I believe that has been my conduct throughout my time with you,” he wrote.