Learn Dutch: De oppositie noemt de nieuwe grondwet erg omstreden. - The opposition calls the new constitution very controversial.

If you want to connect this to a previous thought, you can place niettemin (nevertheless) or evenwel (however) directly after the verb: De oppositie noemt niettemin de nieuwe grondwet erg omstreden. You can also start the sentence with either word, which triggers inversion (verb before subject): Niettemin noemt de oppositie de nieuwe grondwet erg omstreden.

Omstreden means "controversial", but it looks like a verb's past participle. Is it?

Yes, it is the past participle of the verb omstrijden (to fight over or dispute). Used as an adjective, it literally translates to "disputed" or "fought over", which is how Dutch expresses "controversial". Notice that because it comes after the noun it describes (a predicative adjective), it doesn't get an extra -e ending.

Why do we use noemt instead of heet if both can translate to "call"?

Heten means "to be called" or "to have the name" (like Hoe heet jij?). Noemen is an active verb used when a subject gives a name to, or applies a label to, someone or something else. Since the opposition is actively applying the label "controversial" to the constitution, you must use noemen.

Can I use heel instead of erg for "very" in this sentence?