MADRID - Spain's opposition conservative People's Party (PP) is ahead of Prime Minister Pedro Sanchez's ruling Socialists (PSOE) but still short of winning a decisive majority in this month's general election, according to two tracking opinion polls published on Saturday.

To secure an absolute majority of the 350 lower house of parliament seats needed to form a government after the July 23 election, the PP would almost certainly have to ally with the far-right Vox party, opinion polls have shown.

A tracking poll by GAD3 for ABC, a right-wing newspaper, showed the PP could win 37.2% of the vote, or 152 seats, compared to the PSOE, with 28.5% of the vote and 115 seats.

The PP would win more votes than the PSOE and the far-left Sumar party together, the poll found. Sumar would win 25 seats or 11.4% of the vote while Vox would claim 11.7% of the vote or 29 seats.

This would put the PP and Vox combined with just over the 176 seats needed for absolute majority, according to the poll in which 6,501 people were interviewed by telephone from June 29-July 14. The poll had a margin of error of 1.2%.

Meanwhile, the PP's share of the vote fell slightly according to a tracker poll by 40dB for El Pais, a centre-left newspaper.

The PP would win 32% of the vote or 133 seats, one seat down from a previous poll published on Friday, while the PSOE would win 28.3% of the vote, or 109 seats, showing no change on earlier polls.

Vox's share of the vote would stay the same at 14.6% or 39 seats, while Sumar would win 13.8% of the vote or 38 seats, five more seats than a poll published on Friday

The poll for El Pais involved telephone interviews with 2,000 people between 10-14 July, with a margin of error of 2.2%.

The election had been due by December but Sanchez called a snap election after the left fared badly in regional elections in May, apparently hoping to wrongfoot the PP and force it to campaign while also while also negotiating uncomfortable post-local ballot coalition deals with Vox.

(Reporting by Graham Keeley Editing by Frances Kerry)