* Leftist leader appears to gain ahead of vote run-off
* Centrists and leftists join forces around independent PM
By Luiza Ilie
BUCHAREST, Nov 24 (Reuters) - Romania's leftist leader Mircea Geoana saw his chances of winning the Dec. 6 presidential election run-off rise on Tuesday when his party won the backing of a centrist grouping to jointly form a new government.
After an inconclusive first round on Sunday, both Geoana and incumbent President Traian Basescu need to attract voters from the centrists, whose candidate Crin Antonescu came third.
The winner will play a vital role in solving a government crisis that has delayed aid from a 20 billion euro package led by the International Monetary Fund by nominating a prime minister tasked with overseeing deep economic reform.
"We decided to work on an anti-crisis government programme and support Mircea Geoana on the condition we sign an agreement," Antonescu told reporters after a party meeting.
Commentators agree any government built by Geoana and Antonescu would have a shot at political stability after months of bickering that has angered voters and upset financial markets.
But many say such an alliance would be riven by policy differences over painful fiscal reforms and would likely drag its feet on fighting endemic corruption in Romania.
Both Antonescu's Liberal Party and Geoana support a provincial city mayor Klaus Johannis, a member of Romania's ethnic Germany minority, for prime minister.
"We see a competitive advantage for Geoana because of his efforts to unite around a programme with an independent prime minister," said commentator Cristian Patrasconiu.
Almost complete results showed Basescu winning 32 percent of Sunday's vote against Geoana's 31 percent and Antonescu's 20 percent.
The Romanian leu edged up versus the euro on Tuesday in line with regional sentiment as markets awaited more clarity on the election result.
Geoana's Social Democrats (PSD) met earlier on Tuesday and were expected to announce their backing for Johannis later in the day.
"(Johannis) is what Romanians want," senior PSD member Adrian Nastase told reporters ahead of the meeting.
Basescu appeared increasingly isolated after his efforts to invite Antonescu's centrists to negotiate failed.
Striking a confrontational tone that commentators say may cost him the ballot, he said he would not seek support from any grouping.
"Votes are not merchandise," Basescu told reporters in an attack on efforts by Geoana to team up with Antonescu.
The presidential election was an obstacle to political stability in recent months, with Basescu's Democrat-Liberal Party and Geoana's Social Democats unwilling to back each other's efforts to solve the crisis, hoping the vote would give them the upper hand.
Basescu, a tough-talking former ship captain, hopes to attract voters with promises to reform Romania's murky political elites, whom he accuses of soaking up state resources for personal gain.
But his allure as crusader against the pervasive corruption in Romania has waned during his five-year mandate, with voters blaming him for bickering among main political parties that has fanned back-pedalling on reforming the justice system.
Geoana promises social protection for the poorest and state aid at a time of deep recession. (Writing by Justyna Pawlak; Editing by Janet Lawrence) ((firstname.lastname@example.org; +40 21 315 8320; Reuters Messaging: email@example.com))