Unless there is a sudden, unexpected shift in the political winds, Jean-Claude Juncker, a former Luxembourg prime minister, will next week be nominated to the EU’s most high-profile position, president of the Brussels-based European Commission.

The ascension of a largely unknown middle-aged politician from a country less than half the size of the US state of Delaware to a job that only occasionally registers on the radars of global events would hardly seem the stuff of geopolitical crises. But it has opened up old wounds – and created a few new ones – in what has become Europe’s most bitter dispute since the height of the eurozone crisis.

How important is the European Commission president?

Since the eurozone crisis, it has become increasingly powerful. Under the EU’s governing treaties, only the European Commission can propose EU-wide legislation, giving it sweeping powers to control Europe’s policy agenda. Although there are 28 members of the commission – one from each member country – all legislation must be cleared by the president before it is introduced and the president sets the commission’s priorities.

In certain policy areas the commission and its president are the most important decision makers in Europe. For example, it negotiates all European trade deals, rules on all European corporate mergers, and must approve all bailouts of European banks. Although individual commissioners are responsible for these powers, they serve like a cabinet to the president, who has the power to change portfolios and dismiss individual commissioners.

As part of the EU’s emergency response to the eurozone crisis, the European Commission also gained significant powers over national budgets in eurozone countries, including the ability to fine members who fail to live up to tough deficit targets. Although the economic affairs commissioner is responsible for enforcing those rules, the president has ultimate authority.

Beyond its growing power, the commission president also attends all meetings of the Group of Seven industrial powers, giving the president a voice in major global economic and foreign policy issues.

Who is Mr Juncker and why is he so controversial?

Until late last year, when he was defeated in a national election, Mr Juncker was the longest-serving prime minister in the EU, having first assumed office in 1995. But he has been a major player on the European stage for years before that, having served as a key architect of the EU’s 1992 Maastricht treaty – which laid the groundwork for the creation of the euro – when he served as Luxembourg’s finance minister.

He has gained a reputation as an EU fixer for his ability to bridge the Franco-German divide – he speaks both French and German – and in his role as head of the eurogroup of eurozone finance ministers during the darkest days of the common currency’s debt crisis, where he negotiated most bailout deals.

But his longevity has now become a liability. For many EU leaders, particularly Britain’s David Cameron, Mr Juncker has come to symbolise the European status quo, which was roundly rejected in several EU countries in last month’s European Parliament elections, where anti-EU populist parties made unprecedented gains.

Others have raised questions about his management abilities. Eurogroup meetings during the crisis frequently carried on into the early morning hours, and his successor, Dutch finance minister Jeroen Dijsselbloem, has said Mr Juncker was a heavy drinker – something other officials have acknowledged. Mr Juncker has denied any alcohol problem.

If there are questions about his qualifications, why is he getting the job?

Mr Juncker is the product of something completely new to the EU. The union’s most recent treaty, which went into effect in 2010, requires national leaders to “take into account” the results of the most recent European Parliament elections when nominating the new commission president, who must then be approved by a majority in the new parliament.

Activists in the EU’s pan-European political groups took this admonition and went a step further, arguing that they should each run with a “lead candidate” – known by its German word “spitzenkandidat” – who would both serve as their EU-wide face of the campaign and also become their party’s presidential candidate once nominations were considered.

The centre-left Party of European Socialists, which has not held the commission presidency in a decade, were the first to adopt this policy, picking German social democrat Martin Schulz, the outgoing European Parliament president, to top their ticket in November.

All the other party groups gradually fell in line, with the biggest – the centre-right European People’s party (EPP) – only reluctantly agreeing to pick its own spitzenkandidat at a party convention in March. There, Mr Juncker, who had just lost his job in Luxembourg, outpolled former French foreign minister Michel Barnier.

If all the party groups agreed to this, why is there controversy now?

Not everyone enthusiastically embraced the spitzenkandidaten idea – particularly Angela Merkel, the German chancellor, who had to be convinced by political allies to support both the system and Mr Juncker. Officials who have spoken to her said she assumed the scheme would not bind national leaders, who by treaty are given the power to nominate anyone they want.

But the spitzenkandidat process became a bigger deal than expected in Germany – even though very few other countries paid much attention to Mr Schulz or Mr Juncker. When the EPP emerged as the biggest group after last month’s elections and Ms Merkel attempted to give herself some wiggle room, German politicians and press roundly criticised her for being anti-democratic, forcing her to vociferously back Mr Juncker.

But isn’t this all about Mr Cameron? Why is he so upset?

Mr Cameron’s Conservative party pulled out of the centre-right EPP after he became party leader, arguing it was too accepting of closer EU integration. As a result, he had no say in picking Mr Juncker, whom Mr Cameron regards as the wrong man at a time when the EU desperately needs fresh blood.

Mr Juncker and Britain have a long and complicated history. Then-UK prime minister Tony Blair fought with Mr Juncker over the EU budget in 2007, and the Luxembourger is said to have held a grudge against Britain ever since. British officials also consider him too “federalist” – an advocate of giving up more power to Brussels – though Downing Street itself has argued that the “remorseless logic” of the eurozone is for it to become more centralised.

But Mr Cameron’s objections are as much about the process as the man. If the spitzenkandidaten process takes hold, Britain will lose its ability to block presidents it doesn’t like – as it did in 2004, when it stopped then-Belgian prime minister Guy Verhofstadt, and in 1994, when it vetoed another Belgian prime minister, Jean-Luc Dehaene.

Legally, EU prime ministers only need a weighted majority to select a commission president, but in practice one has never been selected over the objections of one of the EU’s largest member states.

Is it such a big deal if Mr Cameron loses a fight like this in Brussels?

Mr Cameron has taken his knocks in Europe before, most notably when he failed to block the EU’s new fiscal compact treaty imposing tough budget rules on eurozone countries. But he has waged a far higher-profile campaign against Mr Juncker. Defeat is likely to leave a deep and public wound.

British officials have also warned it could make it more likely that British voters decide to leave the EU if Mr Cameron is able to deliver on his promise for an in-or-out referendum in 2017, after he is re-elected. Many EU officials believe it is not an idle threat.

Mr Cameron has vowed to go down fighting, and is expected to force his fellow prime ministers to vote on Mr Juncker’s nomination at an EU summit next week – an unprecedented shift for something that has traditionally been done by consensus.

Many in London and Brussels fear Mr Juncker’s election could be remembered as the beginning of the end of Britain’s membership in the EU.