Prosecutors have arrested an aide to the head of the Democratic party of Japan on charges of illegal fundraising, dealing a heavy blow to the resurgent opposition group’s campaign to wrest power from the long-ruling Liberal Democratic party.

The arrest on Tuesday of the aide to Ichiro Ozawa, DPJ president, sparked immediate speculation the veteran politician might be forced to resign ahead of a general election that must be held by autumn and which the opposition has appeared increasingly likely to win.