go downhill
to become much worse and be in a very bad condition, especially after a particular time or event

[TahlilGaran] Dictionary of Contemporary English ▲

deteriorate to become worse:
Air quality is rapidly deteriorating in our cities.
Living conditions here have deteriorated in the past few years.
get worse to become worse. Get worse is less formal and more common than deteriorate in everyday English:
My eyesight seems to be getting worse.
He’s never been well-behaved, but he’s getting even worse.
go down to become gradually worse – used especially about the standard of something:
The hotel’s gone down since its management changed.
Nick’s teachers say that his work has gone down recently.
go downhill to become much worse and be in a very bad condition, especially after a particular time or event:
During the recession the business began to go downhill.
Her health went downhill rapidly after Christmas.
decline especially written to become gradually worse – used especially about standards of living, education, health etc:
After the war, the standard of living declined.
Over the years, the standard of public transport has declined.
degenerate formal to become worse, or to become something bad instead of something good:
Relations between the two countries have degenerated.
The debate degenerated into an argument between the two sides.
escalate to become much worse very quickly – used especially about fighting, violence, or a bad situation:
Further violence could escalate into a full-scale armed conflict.
The political crisis escalated.
spiral out of control to become worse in a way that cannot be controlled and is extremely serious and worrying:
Police say that the violence has spiralled out of control.
Scientists are worried that global warming could soon spiral out of control.
Costs have been spiralling out of control.

[TahlilGaran] English Thesaurus ▲

go downhill
to gradually become worse.
The area has started to go downhill economically in the last ten years.

We started to argue soon after we got married, and things went downhill from there.

[TahlilGaran] English Idioms Dictionary ▲

go downhill
to become worse and worse
The local economy has been going downhill for many years.

[TahlilGaran] English Idioms Dictionary ▲