Senior Liberals Scott Morrison and Julie Bishop have repeatedly refused to answer questions about how they would vote on same-sex marriage in parliament if a plebiscite was successful.

In a heated exchange on the ABC's 7.30 Report, Mr Morrison repeatedly dodged the question, maintaining his line that he would respect the outcome of a plebiscite on the matter.

"I get to choose the words I use as a politician," he said on Tuesday, when asked why he would not confirm which way he'd vote.

He echoed the words of Prime Minister Malcolm Turnbull, who insists legislation will sail through parliament if the plebiscite is successful.

Asked how he'd vote if his south Sydney electorate opposed same-sex marriage in a plebiscite, Mr Morrison said the legislation should pass if the plebiscite was carried nationally.

But Foreign Minister Julie Bishop revealed the vote of her electorate would play a part in her decision making.

Her vote would depend on how the plebiscite vote looked state by state and electorate by electorate, and would also depend on how the legislation looked.

"I would take my electorate's views into account, but I would also take into account how the plebiscite played out across Australia," she told ABC's Lateline.

"I always have a look at what the legislation says and I will respect the outcome of the plebiscite."

Labor is also dodging questions on what position it would take on a plebiscite if it loses on Saturday.

The opposition leader has instead vowed to make legalisation of gay marriage the first legislative business of a Labor government.

Mr Turnbull says a plebiscite will be held before the end of the year once parliament has passed enabling legislation.

But the coalition almost certainly won't have the numbers in the new Senate and Labor and the Greens are opposed to a plebiscite.

Ms Bishop said Labor should support the plebiscite in parliament if the government is re-elected and respect the mandate it had been provided by voters.

Tony Abbott's former chief of staff Peta Credlin has warned of a "schism" in the coalition over its plebiscite plan, predicting any enabling legislation could fail to pass parliament.