The main opposition Constitutional Democratic Party of Japan (CDP) won big on Sunday night, racking up victories in three Lower House by-elections including a closely watched race in a district that was long a stronghold of the ruling Liberal Democratic Party.

The results are a further blow to the already unpopular LDP and Prime Minister Fumio Kishida. But experts say he has little motivation to call a snap general election because the party would likely suffer heavy losses if he did.

On the other hand, with an eye toward boosting his popularity ahead of the LDP presidential race in September, Kishida might decide to order a Cabinet reshuffle and make party leadership changes once the current session of the Diet, Japan's parliament, ends in June.