The ruling Fidesz party said on Saturday that "for us Hungary comes first", but for opposition Tisza Party leader Peter Magyar Brussels is first.
Fidesz said in a statement that Magyar had no principles and he was always "rubbing up against" those who paid him.
"He is a liar. When he was getting paid by Fidesz, he was praising the government, and he would sit in the front row at Viktor Orban's State of the Nation speeches," the statement said. "Now that Manfred Weber has become his ally, Weber maintains his right of immunity, and he receives money from there, he praises Brussels," it added.
"Peter Brussels betrays anybody. His family, his community and his homeland. We must protect ourselves against the Weber-servant Peter Brussels!".