With the dubious of benefit of having worked in the industry when I was a lot younger and stupider...
Campaigning to ban it at this stage is futile, as previous posters have pointed out the revenue it generates across multiple industries is vast, you would not achieve your goal even remotely - all that might happen is that racing HERE in the UK is ended and horses bred here are shipped to Ireland, to Australia, Japan, USA, Dubai etc etc.
What would make a significant difference to horse welfare is some rule changes.
Ban putting horses under 3 years old into training. Ban them touching a racecourse until 5.
Instantly you cut the 'wastage' that is, horses bred excessively 'just in case they're good enough', many of whom are broken down through early training before they see a track, because the costs of keeping them around until old enough is prohibitive.
You also cut down the numbers breaking down because they're raced too young.
You'd also likely improve the handling/early training of those horses because you cannot simply leave them in a field until 3 then bring them in and start riding them, they'd need some handling and education - that stands them in better stead for a future after racing.
More time to get them properly fit to race means fewer injuries on the track, it means fewer jumping accidents too as there is time to school them properly over fences and its far less likely you'd have wild crazy jumpers people take risks on because their fast or because they have incredible stamina (many of the early fallers in the National are this type, nutters with guts but poor jumping skills).
It would also cost more to have a racehorse if you're not seeing a return on your money for four or five years, which reduces the numbers, sharpens peoples minds about taking care of them (remember with the exception of high value stallions, they're rarely insured because the cost outweighs any payout you'd get!).
Theres various other rules I'd bring in, like stipulations on access to turn out, social interaction with others etc... but this ONE change would reshape horse racing in the UK and in a way that produces higher quality, longer lasting, sounder and happier horses, which would lead other countries to follow suit.
If everyone pushing for an outright ban that will never happen in their lifetimes, pushed for this... it could happen.