The odds are high that Prime Minister Shinzo Abe will be put on the defensive heading into a regular parliamentary session as opposition party lawmakers appear to have plenty of ammunition to use against him.

Despite his stepped-up calls for a parliamentary debate on his long-held goal of constitutional reform, Abe will have to spend some time trying to stem a further fall in his support ratings and contain damage to his administration from a spate of simmering scandals, political experts say.

The key question is when Abe will shift from a defensive posture and call an election.

He has said dissolving the House of Representatives is "not on my mind" but in reality he can do so at any time. Still, many observers and lawmakers believe the next election is still some time off and expect he will play his cards close to his chest.

"Frankly it'll be quite a gamble to dissolve the lower house at this moment," said Yu Uchiyama, a political science professor at the University of Tokyo.

"With support ratings falling after the scandals over cherry blossom viewing and casinos emerged, Mr. Abe will have to wait and see how long the situation continues," Uchiyama said.

"From a broader perspective, though, what's alarming is that confidence among voters has fallen sharply, not only in lawmakers as their representatives but also in politics as a whole," Uchiyama said.

Abe has faced allegations that he used a state-funded cherry blossom viewing party for personal gain after his supporters were found to have been invited to the event, meant to honor people such as athletes and celebrities for their accomplishments every year.

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The murky guest selection process also came under the spotlight due to the long-held government practice of drawing up an invitee list based on recommendations from the prime minister, Cabinet members and the ruling coalition.

With lawmakers returning to the Diet for a regular session on Monday, the government's sloppy handling of official documents is at the center of criticism. The guest lists to the event from fiscal 2013 to 2017 were disposed of without the government keeping track of them, in violation of the law on management of public records.

The country's main opposition parties are united in their calls for Chief Cabinet Secretary Yoshihide Suga, who is responsible for the event, to resign over the breach of the law.

Up to now, the Abe administration has overcome a number of scandals. But this time, it has to deal with multiple scandals at once.

The government's pitch for the introduction of casinos as part of integrated resorts faces growing uncertainty following the arrest on suspicion of bribery of Tsukasa Akimoto, a former member of Abe's Liberal Democratic Party spearheading the initiative.

Opposition parties are also taking aim at former Justice Minister Katsuyuki Kawai and his wife Anri -- both LDP lawmakers who are suspected of overpaying campaign announcers in violation of the law in the upper house election in July.

"There should be ample time for debate in parliament (to get to the bottom of the scandals)," a senior LDP lawmaker said.

The prospect of Abe having to focus on damage control makes the outlook uncertain for the next election -- and revising the Constitution for the first time since it took effect in 1947.

"The irony is that he can dissolve the lower house now and the ruling parties will still win. But he can't do so because the LDP will certainly lose seats due to the scandals and constitutional reform will become more difficult," said Masahiro Iwasaki, a political science professor at Nihon University.

Pro-amendment forces now have the two-thirds majority in the lower house needed to initiate any revision process.

In recent weeks, Abe has repeated his wish to revise the Constitution during his tenure, calling for "substantive results" in a parliamentary debate as he wants to add reference to the Self-Defense Forces.

The LDP is hoping to craft a draft revision to the pacifist Constitution this year, even though the ruling and opposition parties have yet to discuss which part of the supreme law should be changed.

The upcoming 150-day Diet session is scheduled to run through June 17, followed by a gubernatorial race for the Japanese capital and the Tokyo Olympics and Paralympics. Political observers say the best timing for Abe to call an election will be shortly after the Summer Games, roughly a year before his term as LDP head ends in September 2021 and the current four-term term for lower house members expires the following month.

Another determinant is the future of the fractured opposition. Were opposition parties to join forces, this would prevent the anti-administration vote from splitting.

Eyes are now on whether the Constitutional Democratic Party of Japan and the Democratic Party for the People can pull off their merger. The clock is ticking after CDPJ leader Yukio Edano hinted at ending the talks on Monday if the parties do not reach agreement.

Iwasaki, however, expects any merger will only have a limited impact on voters, who still remember the 2009-2012 Democratic Party of Japan-led government. Their disappointment at how that administration turned out has helped keep Abe in power since his return.

According to a Kyodo poll released on Jan. 12, 69.3 percent said they did not expect much from the envisaged merger while 22.8 percent said they had expectations.

Failure to convince voters that the opposition is capable of governing the country will only benefit Abe and the LDP-Komeito coalition, according to Iwasaki.

"That will enable him to seek another term as LDP president without being perceived as having wanted it badly," he said.