The European Union's renewed drive to expand eastwards could face its biggest political obstacle yet if French far-right leader Marine Le Pen wins the 2027 presidential election, potentially derailing accession hopes for Ukraine and the Western Balkans at one of the most critical moments in the bloc's history.

While enlargement has gathered momentum since Russia's invasion of Ukraine, with Brussels seeking to bring Kyiv, Moldova and several Balkan states closer to membership, a Le Pen victory would transform France from a cautious supporter into one of the strongest opponents of further expansion.

Le Pen confirmed on July 7 she will contest the presidency after an appeals court allowed her candidacy despite upholding her conviction for misusing European Parliament funds.

"The campaign begins tonight," she told French television after the ruling, pledging to continue her legal appeals while launching her bid to become France's next president.

The far-right politician has long opposed admitting new members to the EU. Her National Rally (RN) party has softened some of its rhetoric on the European Union in recent years, abandoning talk of leaving the bloc altogether. But its voting record points to continued opposition to deeper European integration.

According to a 2025 European Council on Foreign Relations (ECFR) commentary, although younger figures including Jordan Bardella have moderated the party's tone, "the party still promotes a disruptive foreign policy through its votes”.

The report noted that National Rally members of the European Parliament had "voted categorically against any EU enlargement or for deeper European integration" and had also opposed further aid for Ukraine, arguing that the costs were too high.

Such a shift in Paris would carry enormous weight, and not only because every new EU member requires unanimous approval from existing member states before joining. France already occupies a pivotal position in accession negotiations, balancing geopolitical arguments for enlargement against domestic political concerns. A Le Pen presidency would almost certainly move that balance decisively towards obstruction.

Current President Emmanuel Macron has gradually repositioned France as a supporter of enlargement after years of scepticism, arguing that bringing neighbouring countries closer to the bloc has become a strategic necessity following Russia's invasion of Ukraine. Working closely with Germany, Macron has promoted a model of gradual integration that would allow candidate countries to gain increasing access to the EU's single market and institutions before full membership, while insisting that internal reforms to EU decision-making should proceed alongside enlargement.

France has backed Montenegro's accession efforts, expressed support for Albania's membership ambitions by the end of the decade and argued that Serbia should remain firmly anchored to Europe despite persistent political tensions.

Even under Macron, however, France has often been among the more cautious member states. A recent policy paper by a group of think-tanks experts from DGAP, the Jacques Delors Institute, Clingendael Institute and Carnegie Europe examining enlargement debates in Germany, France and the Netherlands found that "French ambivalence" has deepened ahead of the 2027 election.

"At the moment, French authorities do not want to spark a public debate on enlargement in France," the report said, noting that French public opinion remains among the least supportive of expansion within the EU.

The authors said senior French officials often present a different message in private than in public. "High-level diplomats show a puzzling French ambivalence toward enlargement, speaking positively on it publicly ('Ukraine must join as soon as possible') while questioning each move in the negotiations from behind closed doors."

Under the French constitution, any future EU accession treaty must either secure a three-fifths majority in both houses of parliament or be approved in a national referendum.

The think-tank paper described this as "France's biggest political impediment to fulfilling the promise of enlargement", warning that neither route appears politically straightforward amid growing support for the far right.

The European Commission is already aware that Paris could become "the major hurdle on the road to enlargement", the report said, with Enlargement Commissioner Marta Kos planning greater engagement with France in an effort to build public support. Those concerns would intensify significantly under a Le Pen administration.

Rather than pursuing withdrawal from the EU, Le Pen has proposed weakening its supranational institutions, reducing the powers of the European Commission and returning greater authority to national governments in what she describes as a "Europe of Nations”. That vision would make further enlargement considerably more difficult by reinforcing national veto powers and reducing momentum for closer political integration.

Looking ahead to 2027, the European Policy Centre (EPC) believes Le Pen would begin the presidential race as one of the leading candidates.

"Le Pen will probably remain high in opinion polls and will be the favourite to win a plurality of votes in the first round," the EPC wrote, although it noted that France's two-round electoral system means she would still need to assemble a majority in the runoff.

The EPC also suggested that a joint campaign with Bardella could strengthen Le Pen's electoral prospects. "This two-headed candidacy may prove a strong campaigning machine," the report said, combining Le Pen's appeal to core supporters with Bardella's popularity among younger and more traditional conservative voters.

At the same time, EPC analysts argue that tensions remain within the party over how confrontational it should be towards the European Union. Le Pen must decide "which political line to follow, between her anti-system trademark, including radical opposition to the EU, and Bardella's business-friendly search for respectability", the EPC said. "A balancing act between the two lines would expose her to old questions over the RN's ability to govern."

For Brussels, there are implications that extend well beyond French domestic politics. The European Commission regards enlargement as both a geopolitical response to Russia's aggression and a means of stabilising Europe's eastern neighbourhood. Candidate countries including Ukraine, Moldova, Montenegro and Albania have accelerated reforms in anticipation of closer ties with the bloc, while Serbia remains central to EU efforts to maintain influence in the Western Balkans.

Yet accession remains dependent on unanimous political backing from member states. With Hungary already blocking elements of Ukraine's accession process, a French president openly opposed to enlargement could — at least temporarily — close the window of opportunity for candidate countries that opened following Russia’s invasion of Ukraine in 2022.