When political operatives start saying on the eve of a possibly game-changing contest a “win is a win’’ what they are doing is preparing for a mediocre result.

A ReachTEL poll overnight in the Fairfax media in the Sydney suburban seat of Bennelong might have given some encouragement to the incumbent, Liberal John Alexander, but these polling results remain on the borderline of a margin of error.

In any case, a likely swing against Alexander of more than 5% would spook sitting Coalition MPs. A pendulum shift of 5-7% would cost about 20 of them their seats.

The Liberals won the seat in 2016 without going to preferences with 50.41% of the vote on the primaries.

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The ReachTEL poll gave Alexander a 53-47% advantage on a two-party preferred (TPP) basis.

Newspoll earlier this week had the candidates, Alexander and insurgent challenger Kristina Keneally on a 50-50 TPP split.

PM Malcolm Turnbull’s worries about a bad result in Bennelong are underscored by his rhetoric on the eve of voting: he warned voters of the risks of an Alexander defeat because that would rob his government of its parliamentary majority.

“If John Alexander were to lose this seat, then Bill Shorten would be one step closer to being prime minister, and one step closer to being able to fully engage what he has described as his … class war that he proudly says is good politics,’’ Turnbull said.

The ReachTEL poll sought to measure fallout from the resignation of Labor Senator Sam Dastyari. On the face of it, Dastyari’s departure is not having much effect on the result with polling fairly neutral on the subject.

What is unclear is what effect the Prime Minister’s heavy-handed rhetoric implying Chinese government interference in Australian politics is having on an electorate that represents the greatest concentration of people of Chinese origin in the country.

Tension will build in Liberal ranks over the next 72 hours as votes are cast and counted. The Bennelong byelection is arguably one of the more consequential in the country’s history.