Known for her lectern-thumping speeches and occasional outbursts of child-like humour, the 47-year-old single mother joins Chancellor Angela Merkel at the top of German politics.
Well-known for two decades in her SPD party, Nahles is expected to easily beat the only other candidate, Simone Lange, a former policewoman and mayor of the northern city of Flensburg.
Yet well-wishers are ironically hoping that the party stalwart does worse than her predecessor Martin Schulz in the delegates' vote at an SPD congress in the city of Wiesbaden.
A repeat of his 100-percent backing last year amid a euphoric "Schulz hype" would be seen as a bad omen given that in the end the luckless candidate scored just 20 percent in the September 2017 general election, the party's worst post-war result.
While Schulz's roller-coaster ride in German politics has shuddered to a halt, the task of revitalising the dispirited SPD falls to Nahles, who campaigned strongly for another stint as junior partners to Merkel's conservatives.
The challenge for her labour party now will be to at once govern responsibly and convince its dwindling band of working-class voters that it is still fighting for their interests.
Bareknuckle fighter
Nahles, from the party's left wing, scored some landmark successes under the previous Merkel coalition government, notably in introducing a minimum wage.
When voters declined to reward the SPD in the 2017 elections for such gains, Schulz initially vowed a muscular fight from the opposition benches.
Nahles at the time summed up the SPD's combative spirit against the Merkel government with a street brawler's phrase, telling journalists that "from tomorrow we'll smack 'em in the face".
When it turned out the SPD could rejoin Merkel after all, but drive a tough bargain in the process, she used a kindergarden taunt that loosely translates as "na-na na-na boo-boo".
It was not out of style for Nahles, who once mocked Merkel's party in the Bundestag by performing a slightly off-key rendition of the reality-denying theme song of Swedish children's book hero Pippi Longstocking.
While some find such performances grating, few underestimate Nahles, who, like Merkel, is considered a sharp strategist and hard worker, and a bareknuckle political operator.
When she invigorated her party with a passionate speech in January, the tabloid-style Bild daily paid her the questionable compliment of being "the only real guy" in her party.
'Housewife or chancellor'
Nahles, the daughter of a bricklayer, hails from a small village in the rural Eifel region where she still lives in her great-grandparents' farmhouse with her young daughter and likes to ride a horse at the weekends.
A church-going Roman Catholic, she has described herself as a conservative at heart, albeit one who fights for working-class people.
Nahles wrote in her high school yearbook that one day she wanted to be "either a housewife or chancellor".
She founded her party's first chapter in her home village and, while studying German language and literature, joined the SPD youth wing, which she headed from age 25.
In her career since, she has been a key figure in several crucial power plays and fought former chancellor Gerhard Schroeder's Agenda 2010 welfare cuts, telling him she had no time for "political machos".
Most recently she sidelined the two men who had dominated the SPD, Schulz and former foreign minister Sigmar Gabriel. Both are now watching German politics from the parliamentary backbenches.
Political analyst Matthias Micus of the Institute for Democracy Research in Goettingen questioned whether Nahles, as "a representative of the party establishment", is the best choice to lead a renewal of the SPD.
But when in comes to rebuilding the party's image as the champion of the common people, he said, Nahles is "well suited and credible".
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