Germany’s Social Democrats have confirmed two relatively unknown leftwingers as the new leaders of the party at the start of a three-day conference that will focus on its future in Angela Merkel’s grand coalition.

Saskia Esken, a digital policy expert, and Norbert Walter-Borjans, a former state finance minister, were confirmed as the first SPD leadership pairing in the party’s 156-year history in a verification vote on Friday. The conference, entitled New Era, also resurrected the party’s traditional symbol of a red rose.

The pair used their maiden speeches to pledge an intense dialogue with the Christian Democrats and their sister Christian Social Union party over the future of the coalition. They have called for improvements on welfare and environmental policies, as well as an end to the government’s commitment to stick to a balanced budget – the “black zero”.

But they strikingly rowed back from demanding a renegotiation of the coalition agreement with Merkel’s conservatives, or the withdrawal of the SPD from the government, despite having campaigned forcefully on that issue.

“It is time for an about-turn,” said Esken, calling for an end to the neoliberal labour policies introduced by the former leader Gerhard Schröder, the unpopularity of which many members cite as the reason for a dramatic erosion in the party’s support in recent years.

Walter-Borjans, who has been likened to Robin Hood for his battle against tax evasion, said it was time to loosen the German attachment to balancing the budget in order to invest in Germany’s ailing infrastructure, from schools and universities to its digital future, as well as clawing back control from financial markets. “It’s not democracy that should bow to the markets, rather the markets that should bow to democracy,” he said to warm applause.

Olaf Scholz, the finance minister, who lost out to Esken and Walter-Borjans in the leadership race, having been widely expected to win it along with his partner, Klara Geywitz, congratulated the couple, and said the party had no choice but to stick together if it hoped to claw back some of the many millions of voters the SPD had lost, mainly to the Green party and the anti-immigrant Alternative für Deutschland over the past few years.

“A strong SPD is important because a society without us would be very different,” he said. “But we will only manage this if we stick together … and if we ensure we have a plan how to make it work.”

He said the party needed to recognise its achievements in government, including the introduction of a minimum wage and basic pension, which is in the process of passing through parliament.

The duo, nicknamed “Eskabo” by their supporters, have set themselves the ambitious task of increasing the SPD’s voter support twofold, from the current record low of 15%, within the next year.

Their threat to pull the SPD out of the government prematurely – new elections are not due until 2021 – caused Berlin’s political class to hold its breath over the past week, with political observers at home and abroad braced for an earlier than anticipated departure of Merkel, who has said she will not stand again.

Her conservative alliance has vehemently opposed the SPD’s idea of renegotiating the coalition agreement, which took six months to reach.