Addressing supporters in the Transylvanian city of Braşov, USL leader Victor Ponta, 39, promised to restart talks with international lenders as soon as possible but did not elaborate on the cabinet lineup. USL is an alliance of three parties aiming at breaking the power monopoly of President Traian Băsescu (see background).
"A top priority would be to meet the International Monetary Fund delegation in Bucharest ... They must see proof there is continuity in Romania. 7 May is the most probable date for endorsement by Parliament," Ponta said.
Like other governments in the European Union, ousted Prime Minister Mihai Răzvan Ungureanu's cabinet has faced a wave of public anger against plans for spending cuts and tax hikes.
His government collapsed on Friday when it lost a vote in parliament, two months after taking office.
The collapse, which has raised the prospect of months of political turmoil, prompted the International Monetary Fund to postpone its review of Romania's €5 billion precautionary aid deal pending details on the shape of a new government.
"Our term as new government is limited until the November parliamentary elections. We will stick to our USL programme," Ponta said.
The IMF has extended two loan packages to Romania along with the EU and the World Bank, and the precautionary deal is key to Bucharest's efforts to maintain investor confidence.
Ponta said his cabinet would be made of USL officials who had been "former ministers or deputy ministers".
Some commentators have speculated the USL, which has more than 50% support in opinion polls, could roll back some austerity measures if it takes power.
Ponta told Reuters in an interview in February he would not immediately cut value added tax to 19% from 24% but would aim to cut it gradually to 20% by 2016 if he wins in the November polls.
He had said the USL would also stick with the flat 16% tax rate on income and profit, a change from a previous commitment to a progressive tax policy which many businessmen say deters investment.
The European Union's second-poorest member slashed public sector salaries and raised sales tax (VAT) in 2012 to put its economy on a more solid footing but the measures have hit the poorest as Romania emerges slowly from a two-year recession.
"They have little space for manoeuvre and cannot afford not to stick to the IMF deal," said independent political analyst Cristian Pătrăşconiu. "It will be their first direct contact with the real life and the economic crisis after an expected easy win in Parliament."
Ponta's rise marks a changing of the guard for his Social Democratic party, founded chiefly by former communists and now still a dominant force in the USL.
He told Reuters by telephone on Saturday he now controlled 227 seats in the 460 member parliament. The former cabinet was defeated by the votes of 235 deputies, four more than required to oust the government.
Several MPs from ethnic minorities group are expected to back the new government together with a junior grouping UNPR - a former ally of the coalition ousted from power on Friday.
Some analysts and opponents say he remains under the influence of its founder, Moscow-educated Ion Iliescu.
That may pose a longer term risk of shifting towards a more leftist-minded approach including halting a process of streamlining a bloated public sector and a cautious approach towards much-needed privatisation.