I have to press hard on the brakes to stop car on 2000 Nissan Altima

It started 2 days ago and it happens each time i have to brake.

Asked by for the 2000 Nissan Altima
Most likely there's a problem with the power brake booster. It could be simple, like a collapsed vacuum hose or bad check valve, or could be something like a torn diaphragm in the booster which would then require replacement.
It could also be other problems in the brake system, like a failed master cylinder.
Get it checked ASAP, brakes are a safety item.
If you want to find a Nissan specialist, here's our directory link for you:
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1 more answer
Most likely the brake booster or vacuum to the booster or check valve.