The Coalition holds a crushing lead over Labor in the must-win federal seat of Gilmore, catapulting Liberal incumbent Joanna Gash to an election-winning position after just one week of campaigning.An exclusive Mercury/IRIS poll of 400 voters this week shows Mrs Gash is ahead of Labor's Neil Reilly by 58 per cent to 31 per cent in the primary vote, with Greens candidate Ben van der Wijngaart trailing on 11 per cent.On a two-party preferred basis, Mrs Gash holds a commanding 60-40 lead over Mr Reilly, which would deliver her a hefty 10 per cent margin on August 21 and transform the seat into a Liberal stronghold.Mr Reilly, who entered the race just three weeks ago after the late withdrawal of former candidate David Boyle, said the daunting results would only firm his resolve."In a positive sense it makes you fight harder when it appears you're in a corner; I think we'll just come out fighting," he said.Doorknocking in Barrack Heights yesterday, Mrs Gash said she would not rest despite the encouraging figures."Polls come and go, but people will make up their mind at the ballot box and it will depend how we perform in the campaign," she said."I never feel comfortable."The result will give a boost to the Coalition, which considers Gilmore a must-hold seat if it hopes to form government.IRIS executive director Simon Pomfret said Mrs Gash's personal popularity and Mr Reilly's tumultuous path to preselection had helped stretch the Liberals' lead."Jo Gash has a very high profile, she is the incumbent and she's been around a long time," Mr Pomfret said."Neil Reilly's not had a smooth run to be the candidate for Labor, and that's certainly had an impact."Mrs Gash has an unassailable lead in her traditional Shoalhaven heartland, securing a whopping 67 per cent of the primary vote.Mr Reilly fared better in the crucial new northern tip of the electorate, polling well in the Labor-dominated suburbs of Barrack Heights, Warilla and Albion Park Rail.But in the mortgage belt areas of Shell Cove and Shellharbour, Mrs Gash made inroads into Mr Reilly's lead, capitalising on concerns over interest rates and the economy.The primary vote results are a stark turnaround from the slim 0.4 per cent margin purportedly gained by Labor when Gilmore's boundary was redrawn late last year. If the voting pattern was repeated at the election, Mrs Gash would win with an absolute majority, avoiding a preference distribution.The IRIS poll showed Greens preferences would favour Mr Reilly by three to one. Further independent and minor candidates are expected to emerge, which would slightly dissipate the primary vote.Mrs Gash said many voters still harboured anger over the shock overthrow of former prime minister Kevin Rudd, and said asylum seekers and hospital waiting lists were also key concerns.Mr Reilly said health, the economy and unemployment were high on voters' minds.