There's a German former bookseller, a Finnish Eurocrat, or until very recently, the EU's longest serving prime minister of its smallest country. They all want the plum job – to be the next head of the EU's executive, the European commission. And for the first time, foreshadowing what promises to be the fiercest political battle in Brussels for some time, they are looking to the European parliament to realise their ambitions.

Commission chiefs have always been appointed as a result of horse-trading between national leaders who jealously guard that prerogative and view the key post as too important to be left to the vagaries of Europe's voters.

But the EU's Lisbon Treaty shifted the balance of power towards the European parliament, declaring that national leaders consult the parliament on their nomination, that the choice had to reflect the outcome of parliament elections and that the chamber had to endorse the appointment by simple majority.

When elections are held for the European parliament in May, the winning side – either the mainstream centre-right (currently prevailing) or centre-left – will claim a mandate to make their man (all names currently touted are male) the new head of the commission.

This is terra incognito. It has never happened before. And it promises to get very nasty.

"No one knows the rules of the game this time. There could be quite severe institutional conflicts," predicts a senior diplomat in Brussels.

"It will lead to deadlock and a confrontation of the [EU's] institutions," adds a senior Brussels policymaker.

This system is being tested for the first time this year. The parliament has seized the opportunity to broaden its powers by naming leading candidates in the May election. The demand is that the winning group's contender be hoisted into the commission job, meaning that the parliament and not national leaders has the final say.

The British prime minister, David Cameron, is not pleased. More importantly, Angela Merkel, the German chancellor, is strongly against it but is being forced to accept a named candidate to avoid looking undemocratic. Europe's Christian democrats are expected to name their man in March. Jean-Claude Juncker, EU veteran and until recently prime minister of Luxembourg, and Enda Kenny, the Irish taoiseach, are among the names to watch.

The first into the race to become the next head of the European commission, to be appointed in October, was Martin Schulz, the German social democrat and former bookseller from the city of Aachen on the Dutch and Belgian borders who currently presides over the parliament.

"We want to change the system. You don't vote for institutions, you vote for politicians," he told the Guardian in an interview. "The parliament has the right of proposal and the council [national leaders] doesn't need to take it. It's a political question. There could be a stalemate."

He has secured the endorsement of the centre-left in the chamber, of Germany's co-governing SPD, of France's ruling Socialist party, and of 20 other EU centre-left parties, not including Britain's Labour party, which is neither endorsing nor opposing Schulz.

The liberals have responded by setting up a contest to be settled next month between Olli Rehn, the Finnish commissioner for the EU economy, and Guy Verhofstadt, an arch-European federalist and former Belgian prime minister who heads the liberals in the European parliament.

The Greens reacted by inaugurating an internet-driven primary campaign.

All of this has forced the Merkel-led centre-right to accede and reluctantly deliver a pre-election candidate.

Supporters of this system argue that democracy is the winner, that the EU's notorious "democratic deficit" will be reduced, that the European voter will have a direct say in who eventually leads the commission.

In a recent study, Heather Grabbe and Stefan Lehne, from two prominent Brussels thinktanks, denounced the parliament's campaign as "a pretend democratic choice which could also alienate the public further".

The risk is that voters believe they are supporting a candidate who in the end does not get the job, making the exercise of democratic choices look futile. Turnout at European elections are infamously low, having fallen at every turn since starting in 1979. If the winning contender does not get the job, there will be more questions about why people should bother voting. On the other hand, supporters of the scheme hope it will boost turnout by making the ballot more meaningful and concrete.

But opponents see a vote for a commission president via the parliament as illusory, misleading and irrelevant.

Will a Swede vote liberal because of a Flemish contender? Or a Greek vote social democrat in support of a German? Or a Czech be mobilised to back the Christian democrats because he or she wants an Irishman in the top job? Or will all of them vote because of what they perceive to be going on in their own countries and politics?

"This is a very bad procedure," said the senior policymaker. "It will not give us the best way for the best president of the commission. And I am less and less convinced it will be one of the top candidates who gets the job."