LONDON — Wading into an escalating diplomatic feud, the European Union warned Turkey on Monday that a constitutional amendment to drastically strengthen the president’s powers might harm the country’s longstanding bid to eventually join the bloc.
EU officials also urged Turkey to avoid rhetoric like the recent declarations by President Recep Tayyip Erdogan, who said that Germany and the Netherlands had demonstrated Nazi-like behavior in blocking Turkish officials from campaigning in those countries.
Membership talks with Turkey have been moribund, but the prospect of Turkey’s joining the European Union has also long been dangled as an enticement to closer ties.
The warning, delivered in a statement by two officials of the European Commission, the EU’s executive arm, was the bloc’s strongest collective response yet to a sustained verbal assault from Turkey.
The comments have infuriated officials in Germany and the Netherlands, where Turkish migrants and their families make up a sizable minority.
“It is essential to avoid further escalation and find ways to calm down the situation,’’ said the statement by officials Federica Mogherini of Italy, the bloc’s foreign policy chief, and Johannes Hahn of Austria, the European commissioner for negotiations on expanding membership in the 28-nation bloc.
“The European Union calls on Turkey to refrain from excessive statements and actions that risk further exacerbating the situation,’’ Hahn said.
Also on Monday, the Venice Commission, which advises European leaders and studies democracy under the rule of law, issued a report that said the referendum risked giving an excessive concentration of power to the presidency, harming checks and balances and judicial independence.
The report also expressed concern that the change would take place during a state of emergency that was declared after a failed coup attempt against Erdogan in July.
On March 5, Erdogan accused Germany of engaging in Nazi-like behavior after the government of Chancellor Angela Merkel, citing security concerns, canceled rallies at which Turkish government ministers had planned to urge Turkish voters living in Germany to vote yes in the April 16 referendum.
The remarks infuriated officials in Germany, where awareness of the crimes of the Nazi state is a vital part of national policy.
On Sunday, Erdogan said that “Nazi remnants’’ were in the Dutch government, after it refused to allow two of his ministers to hold rallies in the Netherlands.
His inflammatory comments came at an acute time: Voters go to the polls in the Netherlands on Wednesday to elect a new Parliament, and the Freedom Party led by far-right, anti-immigration politician Geert Wilders is expected to make a strong showing.
At an economic meeting in Munich on Monday, Merkel said she had told Parliament last week that she “rejected all kinds of rhetorical comparisons with Nazism in Germany made by Turkish personalities.’’
“These comparisons are completely misleading’’ and “trivialize the sufferings’’ of the victims of Nazism, she said.
At a news conference in Rotterdam, the Netherlands’ prime minister, Mark Rutte, said he did not think that Turkey was trying to interfere in his country’s elections, but he added that his government would not be intimidated by bluster.
