Brazil's most successful environmental campaigner, Marina Silva, has confirmed she will ally herself with a rival to the president, Dilma Rousseff, in the presidential election campaign later this year.

Silva announced on Monday that she would stand as the running mate of Eduardo Campos, the governor of Pernambuco, in an attempt to unseat a president who has been criticised for a sluggish economy, delayed World Cup preparations and a retreat on efforts to protect the Amazon.

The alliance of the two popular politicians under the banner of the Brazilian Socialist party threatens to erode the leftwing vote that has kept the ruling Workers' party in power since 2003.

Rousseff is still ahead in opinion polls, but her popularity is slipping.

A Datafolha poll last week showed support for the president had fallen six percentage points since February to 38%. The conservative candidate Aécio Neves was second with 16%, while Campos was third with 10%.

If no one secures an absolute majority in the first round of elections on 5 October, there will be a run-off between the two leading candidates three weeks later.

During the last presidential election in 2010, Silva – a former environment minister who was then standing for the Green party – emerged as the strongest challenger to Rousseff with 19 million votes.

She remains popular today as a symbol of a clean, green politics sharply at odds with the corruption and waste that sparked huge street protests last year.

Silva was unable to establish her own party – the Sustainability Network – after a nationwide petition drive last year to secure the necessary signatures was controversially struck down by the electoral court.

To widespread surprise, she teamed up with Campos a few months later, but it was uncertain until recently which of them would run for president. Although Silva polls higher than her new political partner, she has now agreed to stand in the junior position of vice-president.

They form a somewhat odd couple. Campos is a political blue-blood whose grandfather was state governor of Pernambuco before him. Silva, on the other hand, is one of 11 children from a poor rubber-tappers family who worked as a maid before entering university and becoming involved in trade unions.

Both though are former ministers who served in Luiz Inácio Lula da Silva's administration and who have grown disillusioned with the way the country has been managed, particularly since Rousseff replaced Lula in 2010.

Launching their campaign ticket, Campos said: "After three years, Brazil has come to a halt, the Brazilian people have lost hope, and the world has become disenchanted with us."

Silva said their partnership aimed to break the polarised politics of the past.

"This is a historic re-alignment, burying once and for all the old republic," she told a gathering of supporters.

How much of a change this represents remains to be seen. Despite its name, the Socialist party – which also includes the former footballer Romário among its members – is a centrist pro-business grouping that is expected to cut public spending and red tape, while keeping in place popular social welfare policies such as bolsa família.

Critics say the alliance is a more about political pragmatism with the two very different candidates hoping to draw voters that they would not normally appeal to.

For the moment, the election remains Rousseff's to lose. But with the campaign proper yet to start, the situation could change rapidly if inflation creeps higher, the economy slows or the World Cup is marred by protests and disruption.

In that eventuality, there is even speculation that Lula might make a return. Though a distant prospect, that would be another game changer. Silva and Campos both served under the former president and are careful not to criticise his policies even today. Were Lula to stand, their alliance would be tested.

• This article was amended on 17 April 2014. The earlier version said Marina Silva and Eduardo Campos were both former members of the Workers' party; both served as ministers in the Lula administration and Silva was in the Workers' party at the time, but Campos was not.