Reading the European Parliament is a tricky business. The absence of a party whip system means there are constantly shifting majorities that make predicting the outcome of contentious votes difficult.

Sometimes MEPs vote along national lines, sometimes in step with their European political groupings. A back room deal may have been struck for support on a given vote, or else they just get a notion. What way a tight vote goes can depend on how many of the parliament’s 720 MEPs turn up on the day.

European Commission president Ursula von der Leyen will be in South America on Saturday to sign a trade deal with the Mercosur countries of Brazil, Argentina, Uruguay and Paraguay.

The trade agreement was backed by a large majority of national governments last week, but still needs to be ratified by MEPs.

The inking of the deal by the commission president turns up the pressure on that European Parliament vote.

The vast majority of European governments see the EU-Mercosur deal, 25 years in the making, as crucial to open up a big new South American market and strengthen Europe’s hand in an increasingly hostile world.

The opposition of beef farmers and the agricultural lobby, who fear competition from cheaper South American imports produced to lower standards, will be keenly felt by many MEPs. Deputies from France, Ireland and Poland will be under huge pressure to vote against the deal.

The old centrist coalition that traditionally ran the European Parliament has started to break apart. Hard-right nationalist parties and the far right have a lot more influence, following gains in the 2024 European elections.

The European People’s Party (EPP), the largest grouping that includes Fine Gael and other centre-right parties, frequently sides with those extreme right-wing forces to pass or block legislation.

That strategy, pursued by Manfred Weber, the conservative German MEP who leads the EPP group, won’t help him get the Mercosur deal over the line.

Far-right parties have made the controversial trade agreement a point of attack, to undermine support for the EU and centre-ground governments.

French far-right leader Marine Le Pen has seized on its provisional approval to heap more pressure on president Emmanuel Macron’s limping administration.

Law and Justice, the hard-right nationalists who lead the opposition in Poland, and Hungary’s far-right prime minister Viktor Orban, will try to kill the trade deal in the European Parliament.

Italian prime minister Giorgia Meloni, whose support was crucial in delivering a qualified majority of national capitals behind the deal, will probably tell her MEPs to approve the trade pact. The former arch-Eurosceptic politician extracted concessions on Common Agricultural Policy funding as the price of her government’s swing vote.

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German chancellor Friedrich Merz and von der Leyen will be expecting Weber to put together a majority to ratify the Mercosur trade deal.

Spurned by his new friends on the extreme right, Weber will have to rely on votes from Renew, the French-dominated centrist grouping that includes Fianna Fáil, and the centre-left Socialists and Democrats (S&D) grouping. The Greens and remaining left-wing MEPs are opposed to the deal, citing environmental concerns.

Both Renew and the S&D are frustrated at Weber and his EPP colleagues’ habit of siding with the far right when it suits.

One source in the Renew group expected about a third of their members – mostly French, Irish and Belgian MEPs – to vote against the deal. That all narrows the path to a majority.

A cohort of MEPs want to refer the trade agreement to the European Court of Justice, to rule on its compliance with EU treaties, which would bog down its final sign-off for some time.

Dublin MEP Barry Andrews has argued the deal will benefit Ireland economically and supports it, but his three Fianna Fáil colleagues will vote to reject it.

Fine Gael is split. Nina Carberry and Maria Walsh are in the No camp. Regina Doherty supports the deal, due to the economic benefits it will bring Ireland’s open trading economy.

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Seán Kelly, Fine Gael’s longest serving MEP, has not made up his mind. The Ireland South MEP said the deal included solid safeguards to protect beef farmers.

Kelly criticised politicians who had been calling for additional protections without any intention of voting to support a compromise text afterwards.

Officials in the European Commission tasked with gaming out what way the votes will fall feel a majority exists to approve the deal, but a narrow and shaky one.

Europe’s claim to be a serious geopolitical player would be seriously harmed by the parliament scuppering the Mercosur deal at the last hurdle. That big picture stuff probably seems like somebody else’s problem to an MEP under the cosh from local farmers pressing them to oppose the deal. This one will be close.