He is self-righteous and eager to run for a third term even though it's not allowed according to the constitution, so he wants to try to change the constitution.
この場合、次のような言い方ができます。
ーHe is self-righteous and eager to run for a third term even though it's not allowed according to the constitution, so he wants to try to change the constitution.
「彼は独善的で憲法で認められていない３期目に出馬することに意欲的だ。なので彼は憲法を変えたがっている」
to be eager to ... で「…に意欲的である」
to run for a third term で「３期目に出馬する」
to change the constitution「憲法を変える」
ご参考まで！
He is self-righteous and wants to continue in his role for a third term, even though it’s currently against the law. He’ll likely start exploring ways to change the constitution to make that possible.
こんにちは！
ご質問ありがとうございます。
ご質問について、いくつか言い方が考えられますが、例えば、
He is self-righteous and wants to continue in his role for a third term, even though it’s currently against the law. He’ll likely start exploring ways to change the constitution to make that possible.
とすると、『彼は独善的であり、3期目に[意欲的。](https://eikaiwa.dmm.com/uknow/questions/105079/)憲法で3期目の任期は認められていない為、彼は憲法を変える為の方策を練り始めるだろう。』となります。
参考になれば幸いです。
カナダ 
日本 
