Spain's central government is already grappling with a major independence movement in the northeastern region of Catalonia, and the Bildu coalition's move in the Basque Country - one of Spain's 17 semi-autonomous regions - comes just weeks ahead of general elections in December.
"The aim is to be able to legally ask citizens" in the Basque Country "what political, economic and social future they want," Bildu spokesman Hasier Arraiz was quoted as saying in a statement on Tuesday.
The bill, which still needs to be approved for debate before it can go any further, would give the head of the Basque government and local mayors the power to consult their citizens over the future of the region.
In its statement, Bildu did not specifically mention independence or a referendum but pointed to the situation in Catalonia several times.
"It's time to confront (the Spanish state) democratically, they are doing it in Catalonia and we want to do it in Euskal Herria (Basque Country)," said Arraiz.
Catalonia tried to hold a referendum on independence for the region, but judges ruled it was against the constitution since all Spanish people have the right to decide on matters of sovereignty.
In November last year, though, Catalonia defied Madrid and pressed ahead with a symbolic referendum. Turnout was just 37 percent, of which over 80 percent voted in favour of independence.
Bildu only has 21 of the 75 seats in the Basque parliament but has asked for the support of lawmakers belonging to the moderate Basque Nationalist Party to push the bill through.
This is not the first time that authorities in the Basque Country have tried to consult citizens on the region's future.
In 2007, the regional government put forward plans to hold a referendum on the Basque people's right to "self-determination", which was seen as a coded reference to its right to secede from Spain.
But then too, the country's Constitutional Court ruled the move illegal.
Like Catalonia, the Basque County has its own distinct language and culture, and it already has autonomy in a number of areas, including policing and taxation.
For four decades the armed Basque separatist group ETA fought a deadly campaign for an independent Basque homeland, but announced an end to violence in 2011.