Three senior MEPs have called for a vote of no confidence in the European Commission of President Ursula von der Leyen.
Members of Renew Europe said on Monday that they would draw up a motion after the Commission gave the green light to Poland's COVID-19 recovery plan.
The bloc had initially withheld money from Warsaw over the country's failures to uphold the rule of law, but then approved Poland's €35.4 billion recovery plan last week.
The announcement represented a truce in a protracted stand-off between Brussels and Poland over controversial judicial reforms.
But the u-turn generated criticism, including from the liberal Renew Europe group in the European Parliament.
Senior figures in the political group say that Poland has still failed to fully protect the rule of law by reforming its disciplinary regime for judges.
The motion of no-confidence in von der Leyen was signed on Monday by the group's vice-president Luis Garciano, Dutch MEP Sophie in't Veld, and former Belgian Prime Minister Guy Verhofstadt.
"We will withdraw our support ... [if the Commission] continues to refuse to seriously apply the conditionality rules on the rule of law", Verhofstadt said.
"The Commission is fully aware that the remedies announced by the Polish authorities are purely cosmetic," he said in a letter to Renew Europe members.
"Last week’s decision of the Commission to approve the Polish recovery plan is not in line with that task," in't Veld added in a statement.
"The 'milestones' fall short of what has been demanded by the European Court of Justice, and they do not address the refusal to recognise the primacy of EU law and the authority of the ECJ by the Polish authorities."
"President Von der Leyen ... has created a major political problem," the Dutch MEP wrote.
The motion of no confidence will first need to be signed by just 10% of the 705-seat European Parliament before it can be put to a vote.
But the motion of censure must then be approved by a significant two-thirds majority to oust the Commission.
During internal deliberations about approving Poland's recovery plan, five EU Commissioners voted against or voiced reservations about the executive's decision.