Having joined the newly created PSDR after the Romanian Revolution of 1989, Athanasiu was first elected to Parliament as a deputy in 1992, being nominated to the Committee for Labor and Social Protection.
A Minister of Labor in the Victor Ciorbea CDR cabinet, he retained the office under Radu Vasile and his own premiership, being replaced by Smaranda Dobrescu during the leadership of Mugur Isărescu. In 1999, he replaced Sergiu Cunescu at the head of the PSDR, leading it into fusion with Adrian Năstase's Party of Social Democracy, PDSR (the fusion led to the creation of the PSD).
In 2000, he was elected to the Romanian Senate on PSDR lists, representing Bihor County (re-elected in 2004 on PSD lists); he stood on the Committee for Foreign Policies. In 2003–2004, Athanasiu also served as Minister of Education in the Adrian Năstase executive (replacing Ecaterina Andronescu).