The first point to note is that he is President of the European Council. Article 9b of the Lisbon Treaty deals with this body. It states:
1. The European Council shall provide the Union with the necessary impetus for its development and shall define the general political directions and priorities thereof. It shall not exercise legislative functions.
2. The European Council shall consist of the Heads of State or Government of the Member States, together with its President and the President of the Commission. The High Representative of the Union for Foreign Affairs and Security Policy shall take part in its work.
3. The European Council shall meet twice every six months, convened by its President. When the agenda so requires, the members of the European Council may decide each to be assisted by a minister and, in the case of the President of the Commission, by a member of the Commission. When the situation so requires, the President shall convene a special meeting of the European Council.
4. Except where the Treaties provide otherwise, decisions of the European Council shall be taken by consensus.
5. The European Council shall elect its President, by a qualified majority, for a term of two and a half years, renewable once. In the event of an impediment or serious misconduct, the European Council can end the President’s term of office in accordance with the same procedure.
6. The President of the European Council:
(a) shall chair it and drive forward its work;
(b) shall ensure the preparation and continuity of the work of the European Council in cooperation with the President of the Commission, and on the basis of the work of the General Affairs Council;
(c) shall endeavour to facilitate cohesion and consensus within the European Council;
(d) shall present a report to the European Parliament after each of the meetings of the European Council.
The President of the European Council shall, at his level and in that capacity, ensure the external representation of the Union on issues concerning its common foreign and security policy, without prejudice to the powers of the High Representative of the Union for Foreign Affairs and Security Policy.
The President of the European Council shall not hold a national office.
So the Council is there to set the political direction and priorities of the Union, but has no legislative power. The Council is thus effectively an executive body, which is there to set the agenda, with the Commission creating the relevant legislation and both the Commission and the Parliament together deciding which legislation gets passed. Now, consider that:
- the European Council’s role is to set the general political direction and priorities of the Union,
- the President is responsible for driving forward the Council’s work and for facilitating cohesion and consensus
From this, we can conclude the President job is to ensure that the Council does in fact set the agenda, by driving it forward himself and seeking consensus and cohesion amongst the heads of state. This is actually a pivotal role, comparable in many ways to that of the Prime Minister in Britain, though the analogy is only a rough one since executive power is distributed a bit differently in the two cases.
Nevertheless, he who gets to set the agenda gets to decide what gets prioritised and what does not, what proposals go forward to be turned into legislation, and what does not. A clever, politically astute operator can do this in a manner that expands the power of the office, exploiting divisions between the other political actors involved and ensuring his goals get achieved. Whether Van Rompuy is such an operator remains to be seen, but the possibility is clearly there.