A leading European Union official is in Warsaw urging Poland's leaders to respect the nation's constitutional order, a visit that comes amid an ongoing crisis over Poland's Constitutional Tribunal.

The visit Tuesday by Frans Timmermans, vice president of the European Commission, comes as the EU is investigating whether there have been systemic violations of the rule of law under Poland's ruling conservative party, Law and Justice.

At the heart of the crisis is the Constitutional Tribunal, which has been paralyzed by the party, which won elections last year. Law and Justice has refused to seat judges chosen legally by the previous government, passed legislation that hobbles the court's ability to issue rulings and refuses to recognize a March ruling by the same court striking down the new laws as unconstitutional.