* Still uncertain centrist Lapid can break stalemate
* Israel has held four inconclusive ballots in two years
* Netanyahu's mandate to form government expired overnight (Adds Lapid comments)
By Jeffrey Heller
JERUSALEM, May 5 (Reuters) - Israel's president on Wednesday chose Yair Lapid, a centrist politician and Prime Minister Benjamin Netanyahu's strongest rival, to try to form a new government, but his path to success was still uncertain.
Israel's longest serving leader, Netanyahu, 71, has been fighting to hold onto office through four inconclusive elections since 2019 and corruption allegations that he denies.
President Reuven Rivlin, in a televised address announcing his choice of Lapid, said the former finance minister had the pledged support of 56 of parliament's 120 members, still short of a majority.
"It is clear that parliament member Yair Lapid could form a government that has the confidence of the Knesset, despite there being many difficulties," Rivlin said.
In a statement accepting the nomination, Lapid, who heads the Yesh Atid party, said he aimed to establish a government of the left, right and centre "that will reflect the fact that we don't hate one another".
But Lapid has ruled out serving in a government with Netanyahu, citing the criminal indictment against the prime minister.
The most recent vote on March 23, held while Netanyahu is also on trial on charges of bribery, fraud and breach of trust, yielded no majority for the incumbent or for a loose alliance of rivals from across the political spectrum aiming to topple him.
A 28-day mandate to put together a coalition ran out at midnight after Netanyahu failed to agree terms with potential right-wing partners, opening the way for Rivlin to assign the task to another member of parliament.
Lapid, 57, also has 28 days to try to form a coalition.
Netanyahu, who last lost an election before the turn of the century, was due to make his own address later in the day.
POWER SHARING
A power-sharing agreement to end the political stalemate has been widely mooted, in which Lapid would rotate in office with ultranationalist Naftali Bennett, 49, of the Yamina party.
Rivlin met the two separately on Wednesday, and each put his own name forward to lead coalition talks, the president said.
"I have just spoken to Yair Lapid and informed him that I am entrusting him with forming a government, whether this is a government that he will head at the beginning, or a government headed by someone else first in which he will serve as alternate prime minister," Rivlin said in his address.