Even if approved, the bill would still face a string of legal probes before becoming law.

Passing the law was a condition laid down by the hardline Catalan separatist JxCat party in exchange for its crucial parliamentary support to enable Pedro Sánchez to begin a new term as prime minister in mid-November.

The controversial law will apply to those wanted by the justice system over the failed 2017 Catalan independence bid, first and foremost JxCat’s exiled leader Carles Puigdemont, who was Catalan regional leader at the time and fled to Belgium to avoid prosecution.

Lawmakers will gather from 3:00 pm (1400 GMT) on Tuesday to vote on the text, which should be backed by a majority of 178 within the 350-seat chamber, including the Socialists, their radical left-wing coalition partner Sumar, and the Catalan and Basque regional parties.

Although the bill is expected to pass the first vote, it will face numerous hurdles before becoming law.

The right-wing opposition Popular Party (PP) has vowed to do everything in its power to slow the bill’s passage through the Senate upper house, where it holds an absolute majority.

It has already modified the procedural rules in the upper chamber and will ask for opinions and reports on the bill before amending it and sending it back to lawmakers for a final vote.

The government has “swapped immunity for power” in the “worst version of power”, opposition leader Alberto Núñez Feijóo told 45,000 protesters in central Madrid during the latest demonstration against the amnesty bill at the weekend.

Lambasted for months by members of the judiciary, the bill is also facing a string of legal challenges that could jeopardise its future.

On the eve of the vote, a Barcelona magistrate said he was extending his probe into alleged ties between Puigdemont and the Kremlin to determine whether he had sought Russian support for an eventual Catalan state.

He also said he had found evidence of “close personal relationships” between Puigdemont’s inner circle and Russians “who held diplomatic roles at the time or had relationships with the Russian secret service”.

READ MORE: Judge in Spain extends probe into Catalan leader’s ‘Russia ties’

According to El País daily, the probe could lead to treason charges against Puigdemont that would not be covered by the amnesty law.

A second legal probe is focused on Democratic Tsunami, a secretive Catalan protest group behind the blockade of Barcelona airport in October 2019 after Spain’s Supreme Court jailed nine Catalan separatist leaders over the failed independence bid.

In November, the magistrate in charge of the probe said Puigdemont had a “leadership” role within Democratic Tsunami and the offences he faces “could be classified… as terrorism”.

Last week, the Socialists were forced to amend the amnesty bill to ensure that it would still apply to those accused of “terrorism” as long as it did not involve “a serious violation of human rights”.

Accused by Sánchez’s government of having a political ulterior motive, the magistrate reacted two days later by highlighting the injuries sustained by a police officer during the 2019 unrest in a bid to circumvent the new amendment.