As campaigning began, a poll in the nationwide Asahi newspaper,gave the LDP 21 per cent support among voters, against 40 per cent who say they will opt for the opposition Democratic Party of Japan (DPJ), which already controls the less powerful upper house of parliament.
Yukio Hatoyama, leader of the DPJ is proving popular with voters and has pushed an Obama-style message of change.
Speaking to a campaign rally in Osaka on Tuesday, he accused the LDP of ignoring the interests of ordinary voters.
"Everyone, the day has come to rewrite history," he told a crowd of hundreds of supporters.
"With your power, let's have the courage to start a new chapter of politics with you all at the centre."
Hatoyama has already squared off with Aso in two televised debates, although analysts have been divided over who came out the victor.
The election is taking place as Japan struggles to emerge from its worst recession since the Second World War.
The downturn has taken a heavy toll on jobs and issues over how to revive the world's number two economy are likely to dominate the campaign.
Hatoyama has said his DPJ will put Japan on road to recovery by putting more money in the hands of consumers, freezing tax rises and adopting a diplomatic stance less subservient to the US.
But his policies have been criticised by Aso, who has said the DPJ's plans amount to irresponsible handouts.
"Pork barrelling without strategy will not lead to economic growth," he said on Monday.
Data on Monday showed Japan's economy returned to growth in the second quarter, formally ending the recession.
|PM Taro Aso is battling plunging
poll ratings [Reuters]
However many analysts said the relatively small increase - 0.9 per cent - after tens of billions of dollars in stimulus spending cast doubt on the sustainability of any recovery and said it would probably do little to revive the prime minister's fortunes.
Aso, who dissolved parliament last month, has seen his popularity ratings plunge to record lows because of his perceived failure to lead Japan out of its troubles and a tendency to make embarrassing gaffes.
His LDP has been in power almost without break for more than half a century and, together with a smaller party, held a commanding majority in the 480-seat lower house of parliament before it was dissolved.
Aso, who took office last September, has urged voters to stay with his party, saying troubled economic times cannot be solved by a new and untested leadership.
In an effort to win over a large block of wavering or undecided voters, he has made much of the LDP's economic stimulus packages which he says helped Japan weather the global financial crisis than it might otherwise have done.
"It is the LDP who will protect Japan. It is the LDP who will protect all the people's livelihoods," Aso told a big gathering of party supporters on the outskirts of Tokyo on Tuesday.
"The economy has returned to positive growth and our steps have produced results... I think we need to continue taking steps for the economy."
But even before Aso took power, voter discontent with the LDP had been on the boil after two predecessors quit abruptly after each spending just a year in office.
A scandal over the government's mishandling of millions of public pension records also continues to upset voters, particularly the elderly.
Some voters, however, have expressed uncertainty over the untested Democrats and whether the broadly centre-left party that includes former LDP members, ex-socialists and younger conservatives, would fare any better in delivering policies.