Macron, France's youngest ever president, and Le Pen have a lot riding on the results of the polls which both have pitched as a re-run of their duel for the presidency two years ago.
The last opinion surveys appeared to show the far-right National Rally (RN) with a slight edge over Macron's centrist alliance, including his Republic on the Move (LREM) party. One poll on Friday put the RN on 25 percent, up 1.5 points in a week, with LREM and its allies stable on 22.5 percent.
Analysts say that two years into his five-year term, the EU represents a critical juncture for Macron and might influence whether the 41-year-old can pursue his pro-business reforms domestically.
But his reputation as the champion of more integration among EU member states is also on the line.
“Symbolically, losing European elections in his own country would be seen as a repudiation of someone so pro-European,” said Sebastien Maillard, director of the Jacques Delors Institute think-tank. “What is at stake for Emmanuel Macron is to have an influence in the future European parliament. This is not a given.”
Macron has made no secret of the significance he attaches to the polls, telling regional French newspapers last week that the elections were the most important for four decades as the union faced an “existential threat”. He has jumped into the campaign himself in recent weeks, appearing alone on an election poster in a move that exposes him personally if LREM under-performs.
Sources close to Macron say a bad loss to Le Pen's National Rally could prompt a major cabinet reshuffle, with the job of Prime Minister Edouard Philippe in the balance.
'Did not impose himself'
After re-branding her party — previously known as the National Front — and overhauling its programme, Le Pen has campaigned since January with the head of state in her sights. She sees the chance not only to deal a blow to Macron's faltering presidency, but for her ideas to move further into the political mainstream.
Le Pen has ditched her long-standing policy of wanting to leave the EU — a so-called “Frexit” — and her proposal to abandon the euro common currency. Instead, she proposes unpicking the bloc from the inside, rolling back its treaties and common rules and turning it into a “union of nation states” who act independently. She has been heartened by gains for far-right nationalist parties across Europe.
“Everything has changed,” she told AFP in a recent interview. “Before we were on our own on the European scene… we didn't have any allies. But in the space of a few months, a whole range of political forces have risen up in spectacular fashion.”
But some projections for the National Rally, with its list led by 23-year-old Jordan Bardella, show it could end up below 2014 levels when the National Front came top with 25 percent.
Le Pen has said she would be “disappointed” to finish behind Macron's party and her reputation as party leader is also in the spotlight. Her gaffe-ridden performance in the presidential election in 2017, particularly a bungled televised debate, has not been completely forgotten by some party faithful.
Le Pen has also had difficulties finding allies despite her dreams — and those of Donald Trump strategist Steve Bannon — of forging a pan-European “supergroup” of ultra-nationalists.
The hard-right eurosceptic ruling party in Poland — PiS — has shunned her over her pro-Russian stance, while Hungary's Prime Minister Viktor Orban has also spurned her advances.
'Give some oxygen'
Sources in Macron's party say that if the LREM falls behind the RN all eyes will be on the margin to determine the magnitude of the reaction.
“If there is nothing in it, behind or in front, I don't see a reshuffle. But if we are three to four points behind the RN, or below 20 percent, people within the ruling party will start to ask questions,” said a person close to Macron, who asked not to be named.
“And this will require a change in personnel,” the source told AFP.
A minister, also speaking on condition of anonymity, added: “If we are far behind the RN then things are going to shake. There will be a big reshuffle. I don't see how we can lose the elections” and not change the prime minister.
For Brice Teinturier from the polling institute Ipsos in France, victory would give the government some “political oxygen”.
By AFP's Adam Plowright and Marc Preel