Monday after the leftists dropped negotiations with rival Liberals.
Since an inconclusive parliamentary election on Nov. 30 that gave a wafer-thin lead to the PD-L, the three main parties have negotiated how to share power and any link-up appeared possible.
Divisions centered largely on who would control the post of prime minister, with the groups all pushing for their candidates.
Political analysts said a coalition including the PSD, once tainted by sleaze scandals and slow reforms, would be the least likely to pursue further efforts to fight corruption, an endemic weakness in the new European Union member.
Much will depend now on President Traian Basescu, who nominates the prime minister under Romanian law.
Having close links with the PD-L, he is likely to opt for a politician from this grouping, but he may still prefer a candidate who would seek a deal with the Liberals of outgoing Prime Minister Calin Tariceanu, observers say.
'This coalition (Democrats and leftists) has a chance to be created. But it will mean the fight against corruption is buried,' said Mircea Marian, commentator for Evenimentul Zilei daily.
Romania joined the EU last year, riding a wave of economic and justice reforms implemented by a centrist coalition between the Democrats and the Liberals, after years of slow progress under Socialist rule.
But a feud between Basescu and Tariceanu ruptured the coalition just months after EU accession, bringing reforms to a standstill and raising concerns about Romania's ability to address threats stemming from the global financial crisis.