But while Mr. Abe’s performance has boosted his party’s chances, those prospects have also been helped by the collapse of any credible opposition. The Democratic Party, the strongest opposition party, is still blamed by the public for its perceived mishandling of the 2011 nuclear crisis in Fukushima, and has been unable to regroup since its defeat at the hands of Mr. Abe’s party in elections for Parliament’s more powerful lower house in December. Depending on how badly it does in Sunday’s vote, some analysts say, the party could split apart.

The extraordinary inability of Japan’s antinuclear movement to make any inroads into national politics has also aided the Liberal Democrats, whose pro-business platform calls on Japan to return to nuclear power. For months after the Fukushima disaster, tens of thousands of protesters took to the streets in noisy rallies that called for an end to nuclear power in Japan. Polls show more than half of Japanese voters still at least passively oppose the restart of the nation’s nuclear plants, most of which were idled after the disaster.

“A lot of Japanese got agitated, for sure,” said Ryosuke Nishida, an expert on public policy at Ritsumeikan University in Kyoto. “But the movement has really failed to seize the day.”

For a time, many disgruntled voters seemed to rally around the charismatic mayor of Osaka, Toru Hashimoto, who had seemed to be what many Japanese yearned for: a decisive politician willing to take on Japan’s powerful vested interests, including its nuclear lobby. But Mr. Hashimoto, who founded his own party, seemed to stumble last year after he quietly dropped his opposition to restarting a nuclear plant near Osaka. Then in May, his party seemed to largely vanish as a factor in the current elections after Mr. Hashimoto outraged many voters by appearing to defend Japan’s infamous wartime system of forcing South Korean and other women to provide sex to Japanese soldiers.

If the Liberal Democrats gain control of both houses of Parliament, Mr. Abe will have the firmest grip on power of any prime minister since his mentor, Junichiro Koizumi, who for a time enthralled Japanese voters with his similar mantra of revitalizing Japan’s sluggish economy. After Sunday, Japan is not legally required to hold elections in either house until 2016, potentially giving Mr. Abe a three-year run before facing voters again.

Analysts say that gives Japan its best prospect in a long while of ending a streak of politically short-lived prime ministers; since Mr. Koizumi stepped down in 2006, Japan has had seven leaders, including Mr. Abe.

This time, the Liberal Democratic Party has put up a remarkably united front, because its lawmakers know that they must ride Mr. Abe’s coattails to win the election. But once elected, they may find fewer reasons to rally around Mr. Abe and more reasons to pursue their own agenda, experts say, possibly hurting Mr. Abe’s ability to pass painful economic changes, or push a right-wing agenda that could include reforming Japan’s pacifist Constitution. The “L.D.P. always has the biggest problems after they’ve won a big victory,” said Tomoaki Iwai, a politics professor at Nihon University in Tokyo.