The EPP, currently the largest group in the European Parliament will choose its candidate for President of the European Commission at its congress, which is being held in Helsinki.
Another large group, the ECR will endorse its candidate next Monday.
Here's an explainer of the Spitzenkandidaten system used in the European Commission presidential nomination process.
What is a 'Spitzenkandidat'?
The word literally means top, or lead candidate in German. The plural is Spitzenkandidaten.
Candidate for what?
The Spitzenkandidaten will compete to become President of the European Commission, the post currently held by Jean-Claude Juncker from Luxembourg. He was the European People’s Party (EPP) candidate in 2014.
How does the system work?
First, choose a candidate: Each political grouping in the European Parliament chooses a Spitzenkandidat by consensus (less likely) or by running a competitive primary-style election process (more likely).
The candidate needs to be chosen ahead of the European Parliament elections, which are being held from 23-26 May 2019.
Following the European Parliament (EP) election, the nominated candidate from the group with the most MEPs is considered by the heads of Members States’ governments, the European Council.
Voting by the European Council to endorse the Sptizenkandidat from the biggest EP group is done by qualified majority voting (QMV).
A qualified majority means 55% of Member States representing at least 65% of the population.
And that’s it?
No. If the candidate is endorsed by the European Council, their name then goes back to the European Parliament for final approval.
Does it have to be done this way?
No. The Spitzenkandidaten system is the current 'best guess'-type interpretation of Article 17 (7) of the Lisbon Treaty, which came into force in 2014.
It lays down the procedure for the election of the European Commission President.
Article 17 (7) says "Taking into account the elections to the European Parliament and after having held the appropriate consultations, the European Council, acting by a qualified majority, shall propose to the European Parliament a candidate for President of the Commission".
In theory (and in theory only), the heads of state could try to ignore the Spitzenkandidat of the largest group in the Parliament and put forward their own nominee.
This would put the Council into conflict with the largest groups in Parliament and make it nearly impossible to secure the Parliament’s backing for their own alternative nominee.
What do supporters of the Spitzenkandidaten system say in its favour?
It’s more democratic: There has been concern among citizens and officials in the EU about a disconnect between institutions in Brussels and Strasbourg and citizens in member states, sometimes called the ‘Democratic Deficit’.
The Spitzenkandidat system attempts to address this by tying the choice of European Commission President to the outcome of European Parliament elections.
This, it is argued, is preferable to the President being nominated by heads of government for approval by parliament. The current Taoiseach is a supporter of the process.
What do critics say?
It’s not merit-based: Amongst other reasons, critics say the process is not the best way to pick the best candidate based on their competence.
Another criticism is that the nominating role of the heads of state in the European Council is effectively by-passed and the subsequent approval by the European Parliament is a rubber-stamping exercise.
Some critics fear that a future parliament election in which populist eurosceptics made large gains would result in extreme elements taking the role of European Commission President.
French President Emmanuel Macron is critical of the Spitzenkandidat system.
What are the pros and cons of taking part?
The con is you could lose. But even that could also be a partial pro: Apart from having a tilt at the top job in the European Commission, competing for a Spitzenkandidat nomination also raises candidates' profiles for potential nomination to other jobs.
A much sought after position which will be filled in 2019, for example, is the role of EU’s foreign policy chief, the High Representative of the Union for Foreign Affairs and Security Policy.
The four largest blocs and their candidates:
European People’s Party (EPP)
Current strength: 219 out of 751 seats
The successful 2014 nominee of the centre-right Christian Democrat group (of which Fine Gael is a member) was Jean-Claude Juncker.
Manfred Weber is an MEP from Germany who was first elected to the Parliament in 2004.
A qualified engineer, he is a member of the Christian Social Union, the Bavarian sister party of Angela Merkel’s Christian Democratic Union.
Mr Weber played the guitar and sang in a band called The Peanuts. Mr Weber is regarded as the leading candidate.
Alexander Stubb is a former Finnish Prime Minister who will need all the stamina he has built up competing in Ironman triathlons to secure the EPP nomination.
He is currently Vice President of the European Investment Bank and has served as both Finance and Foreign Minister in Finland.
The final decision will be made tomorrow at the EPP Congress in Helsinki.
PES (Party of European Socialists) Socialists & Democrats
Current strength: 187 out of 751 seats
The PES's candidate in 2014 was Martin Schulz MEP. The bloc is made of up Social Democrat parties in Europe, including Ireland’s Labour Party.
Independent MEP Nessa Childers is a member of this group in the Parliament.
The European Union's deputy chief executive Frans Timmermans will lead the centre-left campaign in May's EU parliamentary election, after fellow commissioner Maros Sefcovic stepped aside earlier this week.
Mr Timmermans, a former Dutch Foreign Minister is currently First Vice-President of the European Commission with responsibility for Better Regulation, Inter-Institutional Relations, the Rule of Law and the Charter of Fundamental Rights.
Mr Timmermans is also a former diplomat who worked in the Dutch embassy in Moscow.
Mr Sefcovic, who is vice president for energy on the European Commission, said he endorsed Mr Timmermans.
Mr Sefcovic, from Slovakia, said he stood down in the interests of party unity, avoiding a need for the PES to vote on who will lead its campaign at its congress in Lisbon on 7-8 December.
European Conservatives and Reformers (ECR) Group
Current strength: 73 out of 751 seats
The ECR describes itself as "centre right and euro-realist", believing that too many decisions are currently taken at EU level.
It counts the UK Conservative Party (until Brexit in March 2019), the Ulster Unionist Party (until Brexit in March 2019), Poland’s Law and Justice Party and the Swedish Democrats among its members.
Ireland South Fianna Fáil MEP Brian Crowley remains a member, while the rest of the party’s MEPs sit with ALDE (see below).
There is currently one nominee from the ECR group. The group did not put forward a Spitzenkandidat in 2014 because it disagreed with the process. In 2019 it is taking part.
As he is the only nominee, the ECR group will endorse Czech MEP Jan Zahradil on 12 November in Prague.
Mr Zahradil is a member of the Civic Democratic Party and has been an MEP since 2004.
Alliance of Liberals and Democrats for Europe (ALDE)
Current strength: 68 out of 751 seats
The ALDE, of which Fianna Fáil and Independent MEP Marian Harkin are members, has not yet confirmed names of potential nominees.
The party has until February 2019 to finalise its candidate, but it is critical of the Spitzenkandidat process.
The bloc’s leader Guy Verhofstadt, a former Belgian Prime Minister, was the candidate in 2014.
There is speculation as to whether Mr Verhofstadt will put himself forward again next year.
The group is currently in discussion with Emmanuel Macron’s Le Republique En Marche co-operating in a pro-EU campaign in the 2019 Parliament elections.