Juan Jose Ibarretxe, whose push to gain virtual independence from Madrid is causing problems for Spanish Prime Minister Jose Luis Rodriguez Zapatero, would win a referendum on his plan in the Basque Country, the poll also showed.
The findings, published by newspaper El Pais, represent bad news for Zapatero, who hopes his Socialists can oust Ibarretxe's ruling coalition in the 17 April regional polls, burying his troublesome plan for Basque "free association" with Spain.
Ibarretxe champions the plan as a solution to decades of separatist violence in the Basque Country, but Zapatero denounces it as a breach of Spain's 1978 Constitution.
Spanish PM Zapatero
denies he's taking a softer line
The study by Instituto Opina found Ibarretxe's Basque Nationalist Party (PNV) and two allied parties would increase their majority in the regional assembly to between 39 and 41 seats out of 75.
They currently hold 36 seats.
The poll found the Basque Socialist Party also gaining support, with a projected 18 seats from the current 13, but at the expense of the right-leaning Popular Party (PP), which like the Socialists opposes the Ibarretxe plan.
Asked which way they would vote if a referendum were held on the plan - something which Ibarretxe wants but the central government rules out - 47% of those polled said they would vote in favour and 27% against.
The national parliament overwhelmingly rejected the Ibarretxe plan on 1 Febuary, but the defiant Basque leader has pressed on regardless. The next day, he called early Basque elections and vowed if he won to call a referendum.
Accusations on Zapatero
Ibarretxe's challenge has caused political heartache for Zapatero, whose determination to maintain dialogue with the Basque premier has left him vulnerable to PP leaders who accuse him of being soft on separatists.
Zapatero has held out the prospect of reforming the Basque region's existing statute of autonomy, which includes a wide range of powers such as running its own police, but his vague promises have failed to satisfy the restive Basques.
The issue of the Basques' troubled relations with Madrid raises the political temperature in Spain like no other, as it is played out under the shadow of violence by the separatist guerrilla group ETA.
Branded a terrorist organisation by the European Union and the United States, ETA has killed more than 800 people since 1968 in a bombing and shooting campaign for independence.