But with just hours to go before the polling stations open for the House of Councillors election, the signs are not encouraging, and the bleakest scenario being outlined by political commentators is that Koizumi may be forced to submit his resignation to the Diet (parliament) first thing on Monday morning.
There are 121 places in the 247-seat Upper House being contested, and Koizumi may regret publicly setting a definitive target of 51 seats at the outset of his campaign.
At an emergency meeting of the ruling Liberal Democratic Party late on 5 July, top officials set their sights somewhat lower, saying that 44 seats would be the minimum requirement - the same number the party won in the 1998 election. For that disaster, Ryutaro Hashimoto was forced to step down as prime minister.
"I believe the LDP will get 47 or 48 seats and that Koizumi will stay on. But because he will have less than the 51-seat target, he will face some grave consequences," said Takao Toshikawa, an author and political commentator.
"His ability to control and direct the party will diminish," he said, adding that Koizumi might not be able to have everything his own way in a cabinet reshuffle scheduled for September.
An LDP candidate in Yokohama
handing out political leaflets
And while anything more than 48 will leave Koizumi in fairly good political health, any slip the other way - to the 45- to 47-seat level - could prove fatal to his administration, Toshikawa suggested.
In that event, he predicted, former chief cabinet secretary, Yasuo Fukuda, might initially set up what he termed "a kind of interim cabinet that is not part of the political process", to act in parallel with Koizumi's administration and with the blessing of the prime minister until the party can rubber stamp his appointment.
"Fukuda has supported Koizumi for three years and is highly regarded for his prowess, his ability to control the bureaucrats and his ties with the US administration," said Toshikawa.
Should this scenario play out, he said, the main obstacle to Fukuda becoming prime minister might be internal dissent over the fact that three consecutive leaders of the country had come from the LDP's Mori faction.
The only remaining possible outcome is too terrible to contemplate for a party that has ruled Japan almost uninterrupted since the end of the second world war. But a return of 45 seats or less would also be a catastrophe for the country as a whole, Toshikawa says.
"If they get less than 45 seats, the entire cabinet will have to resign and Koizumi will probably step down as well," he said.
"Koizumi has made a lot of mistakes during his tenure, but even then the majority of people supported him and I'm not quite sure why they are suddenly turning away from him"
Professor of politics,
Hokkaido Gakkuen University
The subsequent politicking for the leadership of the party and the likely finger-pointing and apportioning of blame for the rout "will have grave consequences for Japan because politics will grind to a halt, the top posts in the party and the government will be void and the economy will be on hold".
And while the LDP's politicians accept that they have a fight on their hands, they are confident that this most gloomy of predictions can be avoided.
Ichida Yamamoto, who has campaigned on behalf of a colleague in his Gunma Prefecture constituency, says the going is even tough in an area that has always been a stronghold for the party.
"Three prime ministers have come out of Gunma in the last 20 years, but this time around it's much more difficult," he said. "Anything can happen before polling day, but I believe it will be very difficult for us to secure 51 seats.
Koizumi would suffer a fatal blow
if LDP's seat losses exceed 46
"The Democratic Party of Japan have put up a young candidate in this constituency and they are challenging strongly across the country."
With opinion polls putting the approval rating for Koizumi's administration below 40 percent for the first time since he came to power in April 2001, Yamamoto believes a combination of factors have played into the DPJ's hands and made it more appealing to the electorate.
"There are several issues, but it is especially the failure to explain fully to the public the pension-fund reforms and the decision to commit the Self-Defence Forces to Iraq that have been damaging," Yamamoto said.
More fundamentally, however, Japanese politics has evolved into a straight two-party system in which the DPJ has become the LDP's counterpart and is the new home for any disaffected voter.
"Koizumi will stay, of that I'm sure," said Yamamoto. "This is not a general election so even if the result is very bad for the LDP, there will be no handover of power to the opposition.
"As well as that, no one can take over Koizumi's reforms and the party needs him if it is to prevail in the next general election," he added. "If he were to quit, it would cause chaos."
Of the 247 seats in Japan's Upper
House, 121 are being contested
Wisely, however, the DPJ is not counting its chickens just yet. "Yes, at the moment it looks like we're doing pretty well, but the final results on Sunday are going to depend on voter turnout," said Motohisa Furukawa, who was busy campaigning alongside DPJ candidates in Nagoya.
"If the turnout is high, I'm sure that we'll win, but if it's low it becomes much more difficult," he said. "We're doing well according to the polls, but I believe the clear message here is that the people are definitely fed up with the LDP.
"It has become clear that Koizumi's emergence in 2001 has just postponed the destruction of the LDP, which lost its credibility among the public years ago," Furukawa said. "But people know that he's a mirage now; he's not a reformer, he's an agitator and the LDP is no longer the party to take Japan in the right direction."
"I believe the LDP will get 47 or 48 seats and that Koizumi will stay on. But because he will have less than the 51-seat target, he will face some grave consequences"
Takao Toshikawa, author and political commentator
Instead, he believes, that task should fall to new DPJ president, Katsuya Okada, who issued the rallying cry "Stop Koizumi, stop the LDP-Komeito administration" on the first day of the campaign.
"Okada has gradually become very popular," Furukawa said. "It's because he has integrity and his behaviour is straightforward and clear, in comparison to that of Koizumi."
There is one possibility that Koizumi's administration may confound all the political pundits and pollsters - although it may prove a double-edged sword.
Pollsters have not been able to factor in the support for the LDP's ally, New Komeito, because the party told all its members to claim they were undecided if they were ever contacted by polling organisations. New Komeito represents the Buddhist religious group Sokka Gakkai and has well over seven million members across the country, virtually all of whom will turn out on polling day.
The decision to send soldiers to
Iraq angered pacifist Japanese
Should New Komeito's influence prove to be the deciding factor, the party may well seek greater influence in the administration's decision-making processes. But because it holds very different views to the LDP over issues such as defence and fiscal policies, debate could very easily grow into disagreement and, in time, New Komeito breaking off the alliance.
Should that happen, Koizumi would be presiding over a minority government and the end would be a matter of when, not whether.