Eh, "major mainstream" is pushing it. They have 0.5% of the seats in the Diet, approximately similar representation in local government, and have never been part of a ruling coalition. Their membership is geriatric and their vote share slowly declining as well. AFAICT, the main reason they command any support is their staunch pacifism/anti-militarism, not economic policy.

Many post-Communist parties in Europe, like Germany's die Linke, have done much better. But unlike the JCP, they've also employed branding consultants and dropped the word "Communist", the hammers and sickles, etc.