Philippine President Ms Gloria Macapagal Arroyo's ruling coalition has taken an early lead as results from national elections trickled in.
But pundits said a high turnout may deprive her of a solid Senate majority.
MS Arroyo had campaigned for a sweep of 13 Senate seats on offer in the elections, but early returns showed candidates backed by former ruler Mr Joseph Estrada were leading in four senatorial races and that administration candidates were ahead in eight.
Analysts said the 13th and final seat was likely to go to a independent candidate.
The tallies, by the officially authorised National Movement for Free Elections, was based on less than 2 per cent of about 30 million estimated votes cast nationwide.
An exit poll by leading pollster Social Weather Stations in all of the country's 78 provinces showed the same 8-4-1 trend for Ms Arroyo's People Power Coalition - one short of the nine seats pundits say she needs for a solid majority.
Political analysts said the high turnout in Monday's polls may well have favored candidates of Mr Estrada's Power of the Masses coalition, denting Ms Arroyo's hopes of a crushing victory in elections seen as a test of her legitimacy.
Ms Arroyo took office in January after the military joined anti-Estrada street protests.