The question of whether the governance of the European Union (EU) lacks democratic legitimacy has been debated since the time of the European Economic Community in the late 1970s. This led in part to an elected European Parliament being created in 1979 and given the power to approve or reject EU legislation. Since then, usage of the term has broadened to describe newer issues facing the European Union. Voter turnout at the elections to the European Parliament fell consecutively at every election from the first in 1979 up to 2014 when it hit a low of 42.54%, before finally rising in 2019. The 2014 turnout figure is lower than that of any national election in the 27 countries of the European Union, where turnout at national elections averages 68% across the EU.

Opinions differ as to whether the EU has a democratic deficit or how it should be remedied if it exists. Some scholars argue that the EU does not suffer from a democratic deficit as it is more constrained by its plural structure of checks and balances than any national polity. The EU is an intergovernmental institutional framework where democratically elected national governments bargain with each other. According to Majone, non-accountable institutions, such as the European Commission or the Court of Justice, are insulated from democratic contestation to achieve greater efficiency and protect minority rights. Some Pro-Europeans (i.e. those in favour of the EU) argue that the European Union should reform its institutions to make them more accountable, while Eurosceptics argue that the EU should reduce its powers and often campaign for withdrawal from the EU.

The phrase "democratic deficit" is cited as having first been used in 1977 by the Young European Federalists in their Manifesto, which was drafted by Richard Corbett. In 1979 it was used by David Marquand in reference to the then European Economic Community, the forerunner of the European Union. He argued that the European Parliament (then the Assembly) suffered from a democratic deficit as it was not directly elected by the citizens of the Community. 'Democratic deficit', in relation to the European Union, refers to a perceived lack of accessibility to the ordinary citizen, or lack of representation of the ordinary citizen, and lack of accountability of European Union institutions. Generally, the term 'democratic deficit' describes when there is a gap between the electorate's idea of what a democracy should be, and their perceived conception of how their democracy is currently performing.

In the European Union, there are two sources of democratic legitimacy: the European Parliament, chosen by the electorates of the individual EU countries; and the Council of the European Union (the "Council of Ministers"), together with the European Council (of heads of national governments), that represent the peoples of the individual states. The European Commission (the executive branch of the Union) is appointed by the two bodies acting together. Democratic legitimacy within the EU can be compared with the dual legitimacy provided for in a federal polity, such as the United States, where there are two independent sources of democratic legitimacy, the House of Representatives and the Senate, and, to become law, decisions must be approved both by one institution representing the people as a whole and by a separate body representing the peoples of the individual states.

The German Constitutional Court referred to a "structural democratic deficit" inherent in the construction of the European Union. It found that the decision-making processes in the EU remained largely those of an international organisation, which would ordinarily be based on the principle of the equality of states and that the principle of equality of states and the principle of equality of citizens cannot be reconciled in a Staatenverbund. In other words, in a supranational union or confederation (which is not a federal state) there is a problem of how to reconcile the principle of equality among nation states, which applies to international (intergovernmental) organisations, and the principle of equality among citizens, which applies within states. A 2014 report from the British Electoral Reform Society wrote that "[t]his unique institutional structure makes it difficult to apply the usual democratic standards without significant changes of emphasis. Certainly, the principles of representativeness, accountability and democratic engagement are vital, but the protection of the rights of minorities is perhaps especially important. The EU is a political regime that is, in one sense at least, entirely made up of minorities."

One assertion of democratic illegitimacy focuses on the role of the European Commission as a non elected institution being the only entity (with some few exceptions) initiating legislation. This criticism has, in turn, been criticized, using comparisons with the situation in national governments where few MP's bills are ever debated and "fewer than 15% are ever successfully adopted in any form", while government proposals "generally pass without substantial or substantive amendments from the legislature". The Commission is reestablished every five years. Individual members of the incoming Commission are nominated by national governments and the proposed Commission is (or is not) approved jointly and severally by the Council of the European Union and the European Parliament. If Parliament passes a vote of censure, the Commission must resign. Such a censure motion has been brought eight times, but never passed. In 1999, an initial censure motion against the Santer Commission was defeated, awaiting the results of an investigation. When the results of the investigation were published, the Santer Commission resigned, forestalling a second censure motion that was expected to pass with a large majority.

In an attempt to strengthen democratic legitimacy, the Treaty of Lisbon provided that the nomination of the President of the European Commission should "take account" of the result of the European parliamentary elections, interpreted by the larger parliamentary groups to mean that the European Council should nominate the candidate (Spitzenkandidat) proposed by the dominant parliamentary group. However, this has also been criticized from the point of view of democratic legitimacy on the grounds that the European Union is not a country and the European Commission is not a government, also having a semi-judicial role that requires it to act as a "referee" or "policeman" rather than a partisan actor. The fear is that a "semi-elected" Commission president might be "too partisan to retain the trust of national leaders; too powerless to win the loyalty of citizens". This, too, is seen as a possibly insoluble problem resulting from the European Union's dual nature, partly an international organization and partly a federation.

The Electoral Reform Society observed polling evidence from Germany which showed that support for the CDU/CSU (EPP group) ahead of the 2014 European Parliament elections was higher than support for the Social Democrats (S&D group) and that there was little difference between their support in the opinion polls for national and European Parliament elections. This was despite another poll showing that S&D candidate Martin Schulz was more popular among German voters than EPP candidate Jean-Claude Juncker. They concluded that "this does not suggest that the majority of German voters are treating the contest as a chance to choose a Commission President." However, they recommended that the candidate model be kept with "a clearer set of rules for future elections."