Ursula von der Leyen's fate hung in the balance Tuesday after she gave the most important speech of her career to convince European lawmakers she is the right person for Brussels' top job.

The 60-year-old German defence minister will replace Jean-Claude Juncker as president of the European Commission if she secures a majority vote in the Strasbourg assembly.

"The secret to happiness is freedom. The secret to freedom is courage. Let's have courage together," she urged lawmakers, citing the ancient Greek historian Thucydides.  

If the conservative mother-of-seven loses Tuesday's vote - and the ballot could be close - Europe faces a summer of institutional infighting between parliament and the 28 EU leaders.

If her victory is close, or secured only thanks to eurosceptic members, the first female head of the EU Commission will be weakened even before she takes over in November.

"It has been mentioned many times, we have only had two weeks to get to know each other. I am here," she said rounding off her case to parliament after a speech that triggered lively debate.

"A speech is just a speech, it's limited," she admitted, urging members to consult a hastily put together manifesto designed to answer demands from conservative, socialist and liberal pro-European parties. 

She promised: "A climate-neutral Europe in 2015. A more social and competitive Europe. A Europe that makes use of its full potential. 

"A Europe that has a new push on European democracy and a strong Europe that protects our European way of life."

Von der Leyen has had only a short time since the 28 EU leaders nominated her to win over the main centre-right EPP, socialist S&D and liberal Renew Europe blocs she hopes will get her the necessary 374 votes.

She was broadly well-received when she tried to reassure MEPs of her environmental credentials and that she would build an inclusive five-year programme.

"I will put forward a green deal for Europe in my first 100 days in office. I will put forward the first-ever European climate law which will set the 2050 target in law," she said.

But Green leaders said her plan lacked specifics and said their group would not back her.