Independent senator Nick Xenophon is threatening to field lower house candidates in key Liberal-held seats in Adelaide, in a move that could help Labor oust ministers Christopher Pyne and Jamie Briggs from their seats.

Senator Xenophon told Fairfax Media he would preference against the Liberals at the next federal election if the Coalition did not honour its promise to build the next generation of submarines in Australia. This could top up any Labor candidate’s vote and potentially propel the opposition to victory in the key marginal seats.

He confirmed he would field candidates in Mr Pyne’s seat of Sturt, Mr Briggs’ seat of Mayo and others including Hindmarsh, which the Coalition won from Labor at the last election. “The whole idea is for the Libs to do the right thing on submarines,” he said.

“I just want them to keep their promise. If they break their promise, they will be broken in the electorate.”

Mr Pyne hit back and urged Senator Xenophon to be more constructive.

One South Australian Liberal described Nick Xenophon as “unbelievably popular” in South Australia. Senator Xenophon fell just short of two quotas in the 2013 election, which nearly cost the Liberals their second spot, currently held by junior minister Simon Birmingham.

The Nick Xenophon Group Senate candidates polled strongly at the last election, falling just five per cent shy of the Liberals’ vote in the seats being targeted.