In a proper political system?

• Inform the electorate about shit the government is doing
• Suing the government in front of an unpolitical, independent court, up to an unpolitical, independent supreme court
• Bargaining with the government on regular laws for whenever the government needs a 2/3 majority (e.g. for constitutional amendments, which should be a regular thing, not a once-in-a-lifetime event)
• Running a shadow cabinet where each shadow minister is completely up-to-date to everything that happens so that in case the opposition wins the next election they can hit the ground running

Just to name a few things.

But since the USA doesn't have unpolitical, independent courts and not even an unpolitical, independent supreme court, and constitutional amendments are exceedingly rare, the opposition is pretty worthless.

Just for reference: If you take out the zero-day fixes (all amendments that were passed within the first year) and the two amendments that cancel each other out (18 and 21), the USA has had 15 constitutional amendments. France had 15 full constitutional rewrites in the same time.