Voting got underway on Thursday in Britain and the Netherlands in the ninth European Parliamentary election, which will continue for three more days in the 26 other EU members.
Here are five things to watch out for:
Populist high tide
A very loose coalition of populist and eurosceptic forces hope to make inroads into the parliament and threaten the mainstream consensus in support of closer EU integration.
Brussels officials fear they could win 200 of the 751 seats, buoyed by strong showings by Matteo Salvini's Italian far-right League and Marine Le Pen's National Rally from France.
But observers doubt the populists' ability to form an effective coalition, and the mainstream centrist blocs could welcome liberal and Green members into a broader alliance to contain the threat.
Turnout doomed to fall
In the first election to the European Parliament in 1979, 62 per cent of eligible voters in what were then nine member states took part.
Since then the number of members has swollen to 28, but turnout has fallen very five years, to just over 42 per cent in the last election in 2014.
Mainstream national leaders like France's President Emmanuel Macron and Dutch premier Mark Rutte have urged voters to turn out to resist the advance of eurosceptic forces.
But another low turnout would be seen by many as further evidence that the concerns of Brussels are increasingly divorced from the popular mood.
Last chance for lead candidates
Europe's national leaders are extremely sceptical of ceding authority to the European Parliament to choose a candidate for Brussels' top job, president of the European Commission.
But if, for example, the centre-right EPP bloc were to emerge with strong support, it would strengthen its case for its lead candidate, Bavarian conservative Manfred Weber.
Germany's Chancellor Angela Merkel has vowed to champion her ally next week when the leaders meet to discuss names for Europe's main posts.
But a low turnout and an unclear result in the case would strengthen the hands of leaders like Macron and Rutte who oppose the entire "spitzenkandidat" process.
Britain's last hurrah
Since the last European election, British voters have decided to quit the Union altogether, and this week's election might be the last for UK parties.
Brexit Day itself has been postponed, triggering a crisis for Prime Minister Theresa May's government, and Nigel Farage's Brexit Party is expected to do well.
British media report that May will step down as leader of the Conservative Party on Friday, opening space for a more eurosceptic premier to take charge before the October 31 deadline.
This in turn, along with the performance of pro-EU parties, will influence the debate in the other EU capitals about whether to accord Britain another Brexit extension.
Hundreds of thousands of mainly young Europeans, inspired by 16-year-old Swedish activist Greta Thunberg, have taken to the streets in recent months to demand action on climate change.
Through school strikes, civil disobedience and large-scale marches, the movements has succeeded in pushing the issue up the agenda for European political debate.
But will this translate into more votes for Green parties in this week's poll?
Probably not, at least continent-wide. The school strikes, for example, were carried out by those too young to vote, and the movement is better organised in some countries than others.
Polls predict the Green bloc in the 751-seat EU parliament will remain at around 50 members but may become more powerful as a kingmaker if the two main centrist parties fall short of a majority.
Voting got under way on Thursday in Britain and the Netherlands in the ninth European Parliamentary election, which will continue for three more days in the 26 other EU members.