The elections, which run through Sunday and take place in all of the European Union's 28 nations, have never had stakes that high.
Europe's traditional political powerhouses - the center-right European People's Party and the center-left Socialists & Democrats - are set to lose some clout and face their strongest challenge yet from an array of populist, nationalist and far-right parties that are determined to claw back power from the EU for their own national governments.
Here's a look at the vote that starts Thursday in the Netherlands and Britain:
A CLASH OF VALUES
This clash of basic values - between Europe growing more united or more divided - has put the continent at a historic political crossroads.
French President Emmanuel Macron, champion of the closer-integration camp, says the challenge at the polls this week is to "not cede to a coalition of destruction and disintegration" that will seek to dismantle the unity the EU has built up over the past six decades.
Facing off against Macron and Europe's traditional parties are Italian Interior Minister Matteo Salvini, Hungarian Prime Minister Viktor Orban, French far-right leader Marine Le Pen and a host of other populist, right-wing or far-right leaders who have vowed to fundamentally upend Europe's political landscape.
Nationalist leaders from 11 EU nations stood together in Milan last weekend - a show of unity unthinkable in previous years from a group once considered to be on Europe's political fringe. Salvini then declared "the extremists are in Brussels," the home of EU institutions, for wanting to retain the status quo.
"We need to do everything that is right to free this country, this continent, from the illegal occupation organized by Brussels," Salvini said.
TAKING FROM THE TRUMP PLAYBOOK
Europe's far-right and nationalist parties hope to emulate what President Donald Trump did in the 2016 U.S. election and what Brexiteers achieved in the U.K. referendum to leave the EU. That is to disrupt the powers that be, rail against what they see as an out-of-touch elite and warn against migrants massing at Europe's borders ready to rob the continent of its jobs and culture.
Standing with Salvini, Le Pen promised the far-right "will perform a historic feat," saying they could end up as high as the second-biggest political group in the EU parliament.
Predictions show that is still extremely ambitious. Projections released by the European Parliament this month show the center-right European People's Party bloc losing 37 of its 217 seats and the center-left S&D group dropping from 186 seats to 149.
As for the far-right and nationalists, the Europe of Nations and Freedom group is predicted to win 62 seats, compared to 37 currently. Such statistics though could be irrelevant as soon as Monday if national parties start shifting to other EU-wide political groups in the 751-seat European legislature which meets both in Brussels and France's Strasbourg.
Orban's nationalist Fidesz party is now in the EPP's ranks, but has been suspended for its anti-EU stance and virulent anti-migration rhetoric. The Hungarian prime minister might well bolt after the election to a new radical-right group, perhaps to be formed by Salvini, Le Pen and other nationalist leaders.
WAR, TAXES, UNEMPLOYMENT
For many among the EU's half billion citizens, the memories of war have vanished and the EU's role in helping to keep the peace for 75 years, a feat for which it won the Nobel Prize, is overlooked.
Yet Europe was body-slammed by the financial crisis a decade ago and struggled through a yearslong debt crisis that saw nations like Greece, Ireland, Portugal and Cyprus get bailouts and produced recessions that slashed the incomes of millions.
Europe's high taxes, stagnant wages and gap between rich and poor are still a sore point, highlighted now by weekly protests by France's yellow vest movement demanding more help for hard-pressed workers. EU nations have also not been able to forge a common approach to migration, fueling inter-bloc tensions, and its impotence in quickly containing a migrant influx in 2015 has propelled a surge of support for far-right and nationalist parties.
"We have a crisis of the European Union. This is a matter of fact," Macron acknowledged.
Experts say he's right.
"There are a lot of people who fear that things potentially are moving in the wrong direction or already have moved in the wrong direction," said Janis Emmanouilidis at the European Policy Centre think-tank in Brussels. "It is a mix of multiple insecurities which, at the end of the day, is pushing people toward those who are coming up with easy answers."
TURNING INTO A POTENT FORCE
Since the first European Parliament election in 1979, the legislature has slowly changed from a toothless organization where over-the-hill politicians got cushy pre-retirement jobs to a potent force with real decision-making powers.
The EU at first primarily regulated farming but now sets international trade policy for all members and even monetary rules for the 19 nations who use the shared euro currency.
The legislature itself affects Europeans' daily lives in thousands of ways: cutting smartphone roaming charges, imposing safety and health rules for industries ranging from chemicals and energy to autos and food, supporting farming, reforming copyright rules and protecting the environment.
There are no cross-border elections this week, just national polls in 28 nations. Each EU nation gets a number of seats in the EU parliament based on its population. Cyprus, Luxembourg and Malta have the fewest seats with six each, while the EU's most populous member, Germany, has 96 seats.
Up until now, EU elections were tepid affairs. Voter turnout slumped to just 42.6% in 2014 - but that could well change this year.
WHICH WAY FORWARD?
The pro-EU side says increasing integration is essential for the EU to survive in a globalized world. Euroskeptics say it robs national identity whenever more decisions are made at EU headquarters in Brussels.
Yet even some mainstream conservatives can have a euroskeptic streak. Czech politician Jan Zahradil, lead candidate for the Alliance of Conservatives and Reformists, is among those seeking to return more control to Europe's national capitals.
"(We want) an EU that is scaled back, that is flexible, that is decentralized," Zahradil said. "(An EU) that respects national governments and that cooperates with them, that doesn't fight them, that doesn't patronize them, that doesn't lecture them."
For the pro-EU side, in a world in which China, the U.S. and Russia are all flexing their political and financial muscles, Macron urges voters to think about the strength and unity that comes from 28 smaller nations working together.
"If you fragment Europe, there is no chance you have a stronger Europe. Unity makes strength," Macron said.
Associated Press writers Lorne Cook in Brussels, Angela Charlton in Paris, Elena Becatoros in Athens and Colleen Barry in Milan contributed to this report.
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