By Sergio Goncalves

LISBON (Reuters) - Portugal's governing centre-right Democratic Alliance (AD) kept its lead in a new opinion poll released on Friday ahead of a May 18 national election, though it was seen falling short of a majority in a parliament that looks set to remain fragmented.

The ICS/ISCTE survey published by Expresso newspaper showed the AD - which came to power in a snap election last year - dropping to 32% support from 33% two weeks ago.

However, their main rivals, the centre-left Socialists, also fell, to 27% from 29%.

It leaves the AD far from a parliamentary majority, which under Portugal's proportional representation system could only be achieved with at least 42% of the vote.

The March 2024 election saw the AD and the Socialists finish neck-and-neck with 28% and 27% of the vote, respectively, which led to a minority AD government.

Jose Tomaz Castello Branco, a political science professor at the Catholic University of Portugal, said polling suggested that "on the day after the election, the situation will not be that different from what it is today".

"That will be the most dramatic result," he told Reuters.

Prime Minister Luis Montenegro failed to win parliament's confidence two months ago after the opposition questioned his integrity over the dealings of his family's data protection consultancy, prompting what will be Portugal's third parliamentary election in as many years.

Montenegro, now in a caretaker role, has denied any wrongdoing.

Far-right party Chega, with whom Montenegro refuses any deals, is polling third at 19% - above the 18% it garnered in last year's election - apparently emerging unscathed from scandals involving several senior party members.

Liberal Initiative is polling at 5%, virtually the same as a year ago, and not enough to give a potential alliance with the AD a full majority.

The poll suggests 12% of voters are undecided.

ICS/ISCTE surveyed 1,002 people between April 25 and May 5. The margin of error is 3.1 percentage points.

(Reporting Sergio Goncalves; Editing by David Latona and Gareth Jones)