Berlin (AFP) - With the popularity ratings of Angela Merkel's favoured successor plunging and the centre-left SPD in disarray, speculation is growing that Germany's next chancellor might hail from the Greens.

The environmental group recorded rocketing support in European elections in late May, boosted by the youth-led Fridays for Futures school strikes urging action against global warming.

With an all-time high 20.5 percent of votes in Germany, the Greens overtook Merkel's junior coalition partner, the Social Democratic Party, coming in second place in the May 26 EU vote.

Since then, several surveys have put it neck and neck with Merkel's centre-right CDU-CSU alliance, with one even giving it a one percentage point lead for the first time.

With the numbers suggesting that Germany's highest office may be within reach for the Greens, the buzz is not whether the party could fill the seat at the chancellery but which of its two co-chiefs -- Annalena Baerbock or Robert Habeck -- could seize the top job.

Habeck, 49, with his day-old stubble and broad smile, has for months challenged Merkel as Germany's most popular politician.

With momentum clearly on the side of the Greens, Stern weekly magazine put the charismatic writer and politician on its cover and asked: "Our next chancellor?"

Conservative daily Die Welt plumped for his colleague, declaring in a page one commentary this week that "the next chancellor is named Annalena Baerbock".

"If the misery plaguing the CDU and SPD continues, the Greens could end up installing the next chancellor, and will for feminist reasons decide in favour of the assertive Madame Baerbock," predicted the newspaper.

"She also understands the economy... At the moment, she doesn't need to offer more, given the sad state of the competition."

- 'Dream team' -

A mother-of-two, Baerbock, 38, has been described by Spiegel magazine as "the boss" among the two Greens co-leaders and the unifying force within the party.

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She once aspired to become a journalist but after an internship with the Greens at the European parliament, decided that politics was her future.

Credited with finding compromises on tough topics, she has also been praised for ensuring that policy implemented by regional Greens squares with the party's national line.

But in public, it is Habeck who gets the popstar welcomes.

A father-of-four, the personable doctor of philosophy is not shy about showing his sensitive side.

As a writer co-authoring books with his wife from their home in the bucolic countryside by the Baltic coast, Habeck admitted that he had a comfortable family life.

But he "started scolding stupid politicians" and finally "pulled (himself) together, drove to a Green meeting and came back as district chairman".

"Since then I've been a politician through and through."

Although different in style, the Greens leaders have been described as a "dream team".

With attention now trained on them, both have sought to deflect the limelight, saying that "we don't want to revolve around ourselves".

- 'Politically clever' -

For now, the pair can afford to hang back as their competitors struggle to turn the situation around.

Merkel's CDU is under pressure in eastern Germany, where the far-right anti-migrant AfD is expected to make big strides in three state elections in September and October.

On a national level, Merkel is also facing heat as Germany fails to meet its own climate target.

While she was once dubbed the "climate chancellor" for pushing renewables while committing to phase out nuclear power by 2022, today Merkel's party is accused of being a party that panders to pensioners in greying Germany.

More troubling for the CDU, party chief Annegret Kramp-Karrenbauer, who had been poised to take over from Merkel when her term ends in 2021, is struggling to endear herself to the young.

Kramp-Karrenbauer's clumsy handling of a youth-led online rebellion has pushed some away and raised questions about whether she is suitable to lead the country.

The centre-left Social Democratic Party (SPD) is meanwhile experiencing a power vacuum after party chief Andrea Nahles resigned following a disastrous showing at European polls.

Die Zeit weekly noted that the Greens' reticence so far to name its lead candidate for the chancellor job is "politically clever" as polls can prove fickle, and popularity could peak prematurely.

But given that the Greens have secured their spot just behind Merkel's centre-right alliance, the party may soon no longer be able to avoid the decision, it said.

"If there is a chancellor duel on television, the broadcasters would probably have no choice but to invite a Green candidate as the challenger of the CDU-CSU."