A "battle bus" is being organised to take campaigners from Hampshire and West Sussex to a demonstration about the proposed South Downs National Park.
The South Downs National Park inquiry reopens in February
A meeting at Hove Town Hall on Tuesday will decide the agenda for the public inquiry into the park's boundaries, which is due to reopen on 12 February.
A government inspector recommended the National Park should go ahead in July, 60 years after it was first proposed.
But he proposed changes to its boundary which left out the Western Weald.
It meant excluding towns such as Liss and Petersfield in Hampshire and Midhurst and Petworth in West Sussex.
Protect from development
Other areas, including Ditchling and Lewes in East Sussex, were among 24 areas taken out of the park, while 30 others were added.
"This is a critical moment in the history of the South Downs," said Robin Crane, chairman of the South Downs Campaign, which is organising the battle bus.
"If the inspector doesn't allow full debate on his proposed boundary changes we may not be able to... convince him to change his mind."
He said the best possible National Park would include the Western Weald, Ditchling and Lewes.
Natural England, which supports the inclusion of the Western Weald, believes it will be harder to protect it from development if it was not part of the National Park.
East Sussex County Council, which originally opposed the National Park, is happy for Lewes to be excluded.
It has said that because national parks decide on planning applications within their boundaries the process is slower, and would be "horrendous" for Lewes.
But it objects to the inclusion of land at Wilmington and an area from Newhaven Tide Mills to the A259.