The woman seeking to replace Jean-Claude Juncker as the European commission president has made last-minute pledges on the climate crisis, Brexit, an EU minimum wage and gender quotas for company boards as she faces a knife-edge vote on her candidacy.

In leaked letters to the leaders of two of the EU parliament’s main political groups, Ursula von der Leyen, who was nominated two weeks ago by the heads of state and government for the top EU post, has sought to win over critical left-leaning MEPs at the risk of alienating some on the right.

The German defence minister’s candidacy is facing opposition from the Social Democrats (SPD) in her own country, the greens and the hard left, among others, in the vote on Tuesday evening in which she needs to secure an absolute majority to become the first ever female commission president.

Some are opposed to Von der Leyen on the grounds that she is not one of the spitzenkandidaten, or lead candidates, who campaigned ahead of May’s European elections to head up the EU’s executive branch.

Q&A What is the Spitzenkandidaten process? Show The Spitzenkandidaten process is a method by which the European Parliament seeks to influence who is appointed as President of the European Commission - a role held from 2014 to 2019 by Jean-Claude Juncker. Under the process, each political grouping in the European Parliament proposes one 'lead candidate' to take the role. The presidency subsequently goes to the political party winning the most seats, or the one that brings together the widest coalition of support for their candidate. However, this is not a formal process set out in any treaty or constitution. The candidate for the role is nominated by the European Council, made up of the leaders of the EU member states. They are able to ignore the Spitzenkandidaten nomination and to chose someone else. The appointment has to be subsequently ratified by the European Parliament though, and therefore choosing an alternative candidate risks provoking conflict between the institutions.

Her patchy record as a defence minister has also been raised by the SPD in a dossier circulated among MEPs as a reason why Von der Leyen is “an inadequate and unsuitable candidate” to lead the commission for the next five years.

Von der Leyen, a close ally of the German chancellor, Angela Merkel, who has the backing of the centre-right European People’s party, will make a speech to the European parliament on Tuesday morning before MEPs cast their vote in the early evening but it is far from clear that she will be able to secure the backing of the 374 parliamentarians needed for her to take the post from 1 November.

Should she fail to win enough votes the EU would be left in uncharted territory, with the parliament pitted against the leaders on the issue of commission presidency, and with few easy fixes available.

Von der Leyen’s selection, after 50 hours of negotiations among the 28 leaders, was just one part of a package in which candidates were also chosen as presidents of the European council and European Central Bank, in which political affiliation, geography and gender had to be balanced. Sources said an emergency summit for 23 July was being planned in case of her rejection.

If Von der Leyen passed the 374 threshold but failed to receive the backing of 400 MEPs there would be questions over her ability to lead the EU’s policy agenda given the fragility of her majority.

In letters to the Socialists and Democrats and the liberal group, Renew Europe, sent on Monday and obtained by the Guardian, Von der Leyen has pledged to give parliament a role in initiating legislation and says she will push forward on stalled progressive policies on climate change, wages and gender equality.

She has committed to setting “quotas on gender balance on company boards”, a policy first proposed by the commission in 2012, and to establishing a “legal instrument to ensure that every worker in our union has a fair minimum wage that allows them a decent living in the country they work in”.

On climate change, Von der Leyen has said the EU needs to be more ambitious on its 2030 target to cut emissions within the bloc by at least 40% compared with 1990 levels, by raising the aim to at least 50% and providing a plan for a 55% cut.

She has backed the proposal from the French president, Emmanuel Macron, for a “carbon border tax” which would hit companies that want to trade in the EU but are based in countries that do not live up to the bloc’s environmental standards.

Von der Leyen, 60, who has previously spoken of her support for the idea of a “United States of Europe”, has also called for greater burden sharing when it comes to immigration, in a thinly veiled criticism of the failure of some central and eastern European states to take in asylum seekers.

On Brexit she insists that the current withdrawal agreement – described as “defunct” by the likely next UK prime minister, Boris Johnson – is the “best and only deal possible for an orderly withdrawal”. On an extension of the UK’s membership beyond 31 October, she says she would be in support “if good reasons are provided”.

Von der Leyen says in her letters that she hopes the “snapshot” on her positions, some of which are retreads of previous proposals from the commission, will reassure her critics, although there is a risk of putting off MEPs within the more Eurosceptic and rightwing European Conservatives and Reformists group, in which Poland’s Law and Justice is the largest party.

Portugal’s socialist prime minister António Costa tweeted that Von der Leyen’s letter contained sufficient reassurance for left-wing MEPs to give her their support, describing them as “positive commitments”.