(Adds mining tax debate) By Rachel Pannett Of DOW JONES NEWSWIRES CANBERRA (Dow Jones)--Australia's ruling Labor government holds only a slim lead over the main opposition Liberal-National coalition of center-right parties, with less than four weeks to go until a general election, according to the latest voter poll published Monday. The widely-watched Newspoll, published in the Australian newspaper, shows Labor leads the coalition by 52% to 48% on a two-party preferred basis--down from a 10-point lead of 55% to 45% a week ago. It comes as Prime Minister Julia Gillard is struggling to shake off the perceived policy shortcomings that led to the ouster of her predecessor, Kevin Rudd, in a Labor party mutiny a month ago. Labor is now at the same point that it was on the weekend before Gillard replaced Rudd on June 24. Australians are due to go to the polls Aug. 21. Illegal immigration, sustainable population strategy and climate change remain thorny political issues for Labor, despite efforts by Gillard to address them in recent days. Adding to the pressure, small to mid-tier miners resumed an advertising campaign against the Labor government's proposed mining profits tax Monday. The decision revives a controversial issue Gillard thought she had dealt with earlier this month when she tied-up a deal with the nation's three biggest mining companies--BHP Billiton Ltd. (BHP.AU), Rio Tinto Ltd. (RIO.AU) and Xstrata PLC (XTA.LN)--to soften the impact of the tax proposal. The Association of Mining & Exploration Companies, which represents mainly smaller miners, said it will start a three-week advertising campaign against the proposed tax changes, even after the government agreed on July 2 to lower the headline tax rate to 30% from 40% and exempt all minerals, except iron ore and coal, from the new levy. The bruising campaign with industry over the tax played a role in Rudd's downfall, after he steadfastly refused to negotiate over the terms of the tax. AMEC argues that the deal brokered by Gillard was weighted in the interests of the big three mining companies. Simon Bennison, the association's chief executive, described it as a "secret deal with three multi-national mining companies to the exclusion of 99% of the industry". But Treasurer Wayne Swan, a key architect of both the original and amended tax programs, questioned the possible political motivations behind the small miners' campaign. "I have nothing but good will for the mining industry. It is a very important part of our economy," Swan told reporters in Perth. "In my home state (of Queensland)...it is elements associated with the Liberal Party that are behind this (anti-mining tax) campaign," he said. The Liberal-National coalition strongly opposes the mining tax, arguing it will "choke the golden goose" by penalizing one of the country's strongest export sectors. Labor also has yet to lock in a clear policy on asylum seekers. Recent moves toward opening a regional processing center in East Timor for refugees who attempt to reach Australia illegally by boat have encountered strong opposition within the East Timorese government. Gillard has attempted to neutralize fears that even legal immigration is putting pressure on infrastructure and ramping up house prices, amid official projections Australia's population will swell to 36 million by 2050 from around 22 million now. She is promising a "sustainable Australia, not a big Australia", focusing her gaze on the election battleground of western Sydney, where many immigrants first settle upon arriving in the country. The coalition announced Sunday plans to slow the rate of immigration sharply should it win government, a policy that Labor quickly attacked, saying it has already set in train moves to slash immigration at an even faster pace. Liberal leader Tony Abbott vowed to cut Australia's immigration intake to about 170,000 people a year by the end of the next Parliamentary term, down from a recent peak of 300,000. Gillard described the coalition's immigration cuts as a "trick" given migrant numbers already are forecast to fall to around 175,000 in the year started July 1 and 145,000 the year after, following recent immigration reforms. Gillard on Friday unveiled some interim green measures to address man-made climate change, seeking to heal a rift with the party's progressive support base after Rudd angered them in April by shelving plans for a carbon emissions trading scheme until at least the end of the current Kyoto Protocol period in 2012. Labor was elected in 2007 in large part due to its pledge to green up the economy. But a key plank of Gillard's new strategy--establishing a 150-strong "citizens assembly" randomly selected off the electoral roll to spend a year considering a new "community consensus" on climate change--met with skepticism among voters and was lambasted by green groups as an excuse for more delays. Monday's Newspoll of 1,720 voters was taken July 23-25 and has a margin of error of plus or minus 2.4 percentage points. On a straight match-up, or primary-vote basis--which counts only voters' first preferences and doesn't include votes redirected from smaller parties and independent candidates--Labor's rating was 40% versus the opposition's 42%, Newspoll said. Gillard still leads Abbott as preferred prime minister, by 50% to 34%. -By Rachel Pannett, Dow Jones Newswires; 61-2-6208-0901; [email protected] (James Glynn and David Fickling in Sydney contributed to this article) (END) Dow Jones Newswires July 26, 2010 03:40 ET (07:40 GMT)