Why is EU torn over Juncker bid for top Commission post?

EU leaders are trying to decide who could best lead the European Commission. It is the most powerful job in Brussels, shaping EU policy in key areas such as economic reform, immigration and ties with other global powers.

The spotlight is on former Luxembourg Prime Minister Jean-Claude Juncker - the candidate of the centre-right bloc which won the May European elections.

Why all the fuss about Mr Juncker?

Jean-Claude Juncker is often seen as a classic Brussels "insider"

Mr Juncker is a veteran of Brussels deal-making committed to deeper EU integration.

He is regarded as a traditional "federalist" by the UK Conservatives and many other national politicians. They want the Commission to act less like a government and to focus more on developing the single market in key areas such as digital technology and energy.

Mr Juncker was one of the architects of the euro and has been at the heart of the controversial bailouts, which saw tough austerity terms imposed on Greece, Portugal and several other debt-laden countries.

He has been a key ally of Germany's Chancellor Angela Merkel in demanding budgetary discipline in the eurozone. But he also defends the Common Agricultural Policy - accounting for nearly 40% of the EU budget - and European "solidarity", that is, transfers from richer countries to develop the EU's poorest regions. Critics say much of that spending should be better targeted or reined in.

Who gets to choose the Commission president?

Jose Manuel Barroso, from Portugal, is stepping down as Commission chief after serving two five-year terms

The European Parliament now has a much bigger say in the decision than it used to.

It is still up to the national leaders - the European Council - to come up with a name, who will then be presented to the parliament for approval.

But the candidate now needs a "double majority" - that is, approval in both the Council and parliament to get the job.

In the Council it has to be a "qualified majority" - so no single country can veto the choice. Each country has a voting weight in the Council, depending on its population - so Germany has more weight than Malta, for example. That is the meaning of "qualified".

If the UK wants to block Mr Juncker it needs enough support from "weighty" countries to swing the vote. That now looks unlikely, after the centre-left leaders of Italy and France said they would back Mr Juncker, lining up with Chancellor Merkel and others.

The Council is expected to make its choice on Friday. UK Prime Minister David Cameron says he will demand a vote by EU leaders if Mr Juncker is nominated. Normally the choice would be made informally, not by formal vote.

Then it is the parliament's turn. And in parliament the candidate requires an absolute majority - 376 votes in the 751-seat assembly. The vote is expected in mid-July.

For the first time the Council has a treaty requirement to make the choice "taking into account the elections to the European Parliament".

A majority in the parliament believes the lead candidates ("Spitzenkandidaten") put forward by the main pan-European parties must be the ones considered by the Council. Parliament can veto the Council's choice, so there is pressure to agree on a compromise candidate.

Why is the Commission president so powerful?

A major EU-US free trade deal could be a big prize for the next Commission chief

The Commission drafts EU laws, which are adopted by all 28 member states after amendments by the Council and European Parliament. Only the Commission can initiate EU legislation - though often it does so at the request of politicians.

The Commission also acts as guardian of the EU treaties - it can levy fines on governments or firms which breach or ignore EU law. And it negotiates far-reaching trade deals between the EU and other countries, affecting millions of jobs.

The Commission's powers increased during the eurozone crisis - in particular, it now vets national budgets before they are adopted, in the so-called "European Semester" procedure.

The famous clash between former UK Prime Minister Margaret Thatcher and former Commission President Jacques Delors in the 1980s highlighted the importance of the president's job. The late Lady Thatcher felt Mr Delors - chief architect of the euro - was trying to turn the EU into a "superstate".

Does the UK have any allies in opposing Mr Juncker?

The big TV debate between the parties' top candidates was a highlight of last month's election campaign

Some national leaders have expressed reservations about Mr Juncker cautiously - but that does not mean they will not back him. It appears that Mr Cameron can count on Hungary, but not necessarily on Sweden and the Netherlands - despite their firm support for the UK stance on the single market.

Mr Cameron wants a figure who will concede some ground on sovereignty - that is, allow more UK opt-outs. That is a priority for him ahead of his proposed in/out referendum on EU membership in 2017.

There is no sign that the European Parliament will accept candidates other than those put forward by the parties. And the runners-up to Mr Juncker are also, broadly speaking, federalists: the Socialists' Martin Schulz and Liberals' Guy Verhofstadt.

Who are the other potential candidates?

The prime ministers of Denmark (L) and Finland are among possible contenders for the job

Various names have been mentioned widely in the media and by national politicians, though they have not been confirmed as candidates.

Some of these may be considered for the other top EU jobs to be filled soon: High Representative for foreign policy (replacing Baroness Ashton) and a new European Council President (replacing Herman Van Rompuy).

They include: Danish PM Helle Thorning-Schmidt, former Italian PM Enrico Letta, Finnish PM Jyrki Katainen, Irish PM Enda Kenny, Polish PM Donald Tusk, Lithuanian President Dalia Grybauskaite, IMF Managing Director Christine Lagarde and the former Director-General of the World Trade Organization, Pascal Lamy.

The EU will also appoint a whole new Commission team later this year.

There is speculation that Mr Juncker might end up with Mr Van Rompuy's job

Last time, after re-appointing centre-right politician Jose Manuel Barroso to head the Commission - it was a classic Brussels compromise to appoint Baroness Ashton - a centre-left woman - as foreign policy chief.

Some countries may prefer to get a powerful commissioner's job, rather than one of the top posts, if it is an area of particular national interest. The UK is reported to be eyeing the internal market and trade commissioner jobs, and Poland may be happy to get the energy portfolio.