Como dizer "Defender uma ideia" em inglês

Using "defend" is not wrong. The sentence would be: We will always defend the idea agaist death penalty .
We must defend what we think it's right/the correct thing to do.
But, you could also use We've got to stand (for st)
Example: We've got to stand our grounf > Nós temos que defender nossa causa
We've got to stand for what we think it's right/the right thing to do. (tradução da frase

Sugestao:

Stand up for sth

Ex: The only crime they've ever committed is to stand up for their rights.

Also:

To stick up for an idea / belief / etc.

"We need to stick up for our rights / for what we think is right".

Conversely, one can "stand against" an idea or belief.

"The party has stood against the death penalty for decades".

Sugestão:

Support,

You support your favourite football team and your those things (something) you think it is worth standing for.

Cheers.