The possibility of sanctions, which is laid down in article 7 of the EU treaty, has never been used.

The commission’s ex-president Jose Manuel Barroso labelled it as the "nuclear option", hoping the mere threat of sanctions would discourage national governments from going rogue.

But Poland's political crisis intensified over the weekend, with opposition MPs occupying the parliament's plenary hall to block its proceedings since Friday.

The EU's scrutiny over Poland already features historic firsts - the commission activating its rule-of-law mechanism earlier this year to probe the situation in the country.

In July, the EU executive found that Poland had failed to guarantee the independence of its courts.

It published a list of recommended actions that would redeem the situation.

Poland dismissed the recommendations as "political interference" and largely ignored them.

A commission source told Polish press agency PAP that the most recent turmoil convinced commission president Jean-Claude Juncker to put Poland on the college's agenda.

'Article 7 or nothing'

The source wouldn't specify what actions the commission could suggest, but according to Laurent Pech, professor of law at Middlesex University in London, the next step in the commission's probe, as foreseen in the mechanism, was triggering article 7.

"It's either that or doing nothing," he told this website.

"Practically speaking, one may expect the commission to put forward a document setting out the case that there is a clear threat of a serious breach by Poland of the values laid down in article 2 of the EU treaty."

The document would be sent to the European Parliament for its consent. The EU Council, representing member states, could then, acting by a majority of four-fifths of its members, give Poland a chance to reply to the commission’s case before addressing recommendations to it.

"In my opinion, the council would then most likely ask Poland to comply with the commission's recommendations of 27 July."

Given how little interest Poland's rulers have shown to living up to recommendations so far, it's likely they will ignore them this time too.

Even so, Poland is unlikely to face sanctions such as losing its voting rights because the move would need unanimity in the council.

Hungary, which has its own rule of law row with the EU, has already vowed to shield Warsaw from consequences.

Some have argued against triggering article 7 on these grounds, as that would only highlight how powerless the bloc is.

"Doing nothing isn't an option," Dutch liberal MEP Sophie in't Veld insisted.

"The commission has the duty to act, even in the lack of support from the council, when there are huge, systemic problems with the rule of law in a member state."

If the commission triggers article 7, it would make Poland a headache for the Council and its president Donald Tusk, a former prime minister of Poland and the rival of governing Law and Justice.

It would be Tusk's responsibility to act, but it's unclear what he would do in the situation.

An EU pact for democracy and rule of law

Sophie in't Veld said the commission should start working on an EU mechanism on democracy, the rule of law and fundamental rights that actually works.

Earlier this year, she steered through the parliament a report calling for such a measure, which would be based on better coordination of existing instruments: the commission's rule of law framework, as well as the council's rule of law dialogue, a reporting mechanism, notification, and sanctions through infringement procedures or article 7.

"It's not just the case of Poland. There are many other examples in the EU showing the growing need for a mechanism to uphold democracy, rule of law and fundamental rights," in't Veld said.

As for Poland, the EU can't do much about the constitutional crisis, without the cooperation of Poland.

"We support the commission's rule of law mechanism, but in the end, any solutions must be made in Warsaw," Ryszard Petru, leader of the Polish liberal party Nowoczesna and in't Veld's colleague, told EUobserver over the weekend.