The European Parliament is directly elected by European citizens, and has full legislative powers, and certain powers of initiative. Here is a brief explanation of how it works.
MEPs from across Member States sit together in political groups along the ideological spectrum, from the nationalists on the far-right, to the radical-left on the other. Labour MEPs are part of the Group of Socialists and Democrats (S&D).
In the EU, the European Commission is responsible for drafting and proposing legislation (article 17.1 of the Treaty of the European Union), but has not power so deciding or adopting legislation. The Commission must act to promote the “general European interest”.
The European Parliament has the power to request that the Commission make a proposal on a subject. If the Commission declines to make a proposal, it must provide its reasoning to the Parliament (article 225, Treaty on the Functioning of the European Union).
The Commission does not simply propose legislation out of the blue, but is required to undertake a thorough process of consultation with stakeholders, civil society, local authorities, the European Parliament, the Council, and perform impact assessments of the legislation. To that end, the Commission publishes policy documents outlining its intentions, such as Communications.
The European Parliament adopts Reports and Resolutions in response to Commission draft proposals, or Own Initiatives Reports on policy areas it feels must be addressed by the Commission and Council. This means there is a continuous conversation between the institutions and legislative proposals are shaped between them.
The Commission therefore knows not to make proposals which will be opposed by the Parliament, or blocked by the Council. In almost all cases, the Commission take a line that reflects the centrist position in the Parliament, sometimes leaning a little to the left (e.g. on civil or human rights and the environment), and sometimes more to the right (e.g. trade and economic policy).
All European legislative proposals are amended and then adopted by the European Parliament and the Council of Minister. The European Parliament has the final say on legislation.
The EU institutions must “practice mutual sincere cooperation” (article 13.2 TEU) in order to truly promote the effectiveness of EU policy-making and promote the objectives and values of the EU. If the Parliament decided that the Commission was in breach of its obligation to sincere cooperation, it could sue the Commission before the European Court of Justice.
In short, the European Parliament must be consulted before the Commission makes any proposal, and the Commission must take the Parliament’s position into account. The Parliament has the power to request a proposal, but the Commission can refuse if it has good reasons. However EU decision-making is much more cooperative and consensual than in the UK.
In practice, in the UK, the government controls Parliament through its Parliamentary majority and party discipline, so that the Opposition in act has relatively little power to propose or shape legislation. In the EU, the Commission and the European Parliament are separate from one another, and the Parliament forms many different coalitions on different issues, so that MEPs can have a disproportionately large impact on legislation compared to MPs that are not in government.