Jobbik, once known for its hard-line and anti-Semitic views, has turned into a milder party in recent years, as it shifted towards the center to challenge Fidesz, the ruling right-wing party of Prime Minister Viktor Orban.
The State Audit Office, headed by a one-time senior Fidesz official, hit it with a 660 million-forint ($2.4 million) fine for illegal spending in 2017.
Now it faces another fine, related to 2018 campaign spending, of 273 million forints. That has dealt the party, still the second strongest in Parliament, a potentially lethal blow, a Jobbik spokesman said.
"It's entirely plausible that Jobbik as a party, and its parliament group, will cease to exist," spokesman Peter Jakab told Reuters. The party has called an extraordinary congress to set its future course.
The state auditors said in a statement on their web site on Thursday that Jobbik used unidentified and illegal campaign funds.