The Iranian parliament has approved a bill seeking to change election law and overturn a ban on reformist candidates.
Angry MPs are holding a sit-in against the ban in parliament
An emergency session of MPs decided to intervene in a crisis sparked by the Guardians Council ban on thousands of candidates from next month's elections.
Under the changes, those approved for past elections would be able to run again unless there is strong evidence to prove they are unfit.
But to become law, the bill must be approved by the Guardians Council.
The vetting body set off what correspondents say is Iran's worst political crisis for years when it barred more than 3,500 mostly reformist candidates from the 20 February parliamentary poll.
It later reinstated about 350 of the nominees after Iran's supreme leader, Ayatollah Ali Khamenei, ordered a review.
IRAN'S REINS OF POWER
Supreme leader Ayatollah Ali Khamenei: Appointed for life, overrides all other authorities (pictured above)
Guardians Council: Half chosen by Khamenei, responsible for vetting election candidates and laws
President Mohammad Khatami: Elected for four years, power can be circumscribed by clerics
Parliament: 290 members introduce and pass laws, subject to approval
But reformist MPs who dominate the legislature remain angry. Eighty of them were deemed unsuitable to seek re-election and a parliamentary sit-in against the council's decision is continuing.
Sunday's session of parliament, broadcast live on radio, classified the election bill as "triple-urgent".
This category is reserved for when parliament feels the basic rights of the nation are in serious jeopardy or the country is in great political or military danger, correspondents say.
It has not been used since the 1979 Islamic revolution.
The BBC's Miranda Eeles in Tehran says MPs want to add two clauses to the election law, effectively forcing the Guardians Council to reverse its rulings.
The first amendment would allow all sitting members of parliament or candidates approved for past elections to run for office unless legal documents proved their incompetence.
The second is aimed at eliminating politically motivated disqualifications by requiring the Guardians Council to approve anyone deemed by local trustees to be loyal to Islam and the ruling Islamic establishment.
Reformist legislator Reza Yousefian said the proposed changes complied with statements by Ayatollah Khamenei, who made an unusual intervention to order the Guardians Council to review its bans and find a solution to the crisis.
"It's a historic test for the Guardians Council, which claims to be following the leader," he told the Associated Press news agency.
"The bill is actually what the leader said in a way a few days ago."
President Mohammad Khatami and senior officials have also spoken out during the row.
Having appealed for calm while others were threatening mass resignations, Mr Khatami hardened his line on Saturday.
He issued a joint statement with parliamentary speaker Mehdi Karrubi demanding that the Guardians Council make a "full review" of their blacklist.
They said the disqualifications and ensuing furore were "against the dignity of the noble Iranian nation", but stopped short of calling for an election boycott.
The Guardians Council has promised to check all the disqualifications and issued revised judgements by 30 January.