She said the 101/102 service was the only public transport on the A702 between the capital, West Linton, Carlops and Dolphinton.
The Stagecoach buses also serve Biggar and Moffat – with service 102 going via Thornhill – en route to Dumfries.
They face being cut back or ended after joint funder Scottish Borders Council decided to axe its share from August.
South West of Scotland Transport Partnership (SWestrans), which is a fellow funder with Strathclyde Partnership for Transport (SPT), has warned passengers: “There is a real possibility that no link will be affordable.”
Carlops-based Stevenson, who appeared in Braveheart, said: “This is a well-used route which takes people to work, school, university, college, training, hospital, GP and dental appointments.
“It is a lifeline service to people in rural communities who might be at risk of loneliness and isolation.
“Apart from people’s livelihoods, it will seriously affect the local economic infrastructure, use of our village halls, hotels, pubs and restaurants.
“It will affect the local economy in so many ways – walkers who come to the Pentlands, and use local shops and pubs.
“Scottish Borders Council must protect and improve the link between these communities and Edinburgh. Anything else would be unacceptable”.
Tweeddale West SNP councillor Heather Anderson, who has launched a petition against the cuts, said: “Public transport is already decimated in this area so any further reduction would have a disproportionate impact.”
An SPT spokesperson said: “The decision by Scottish Borders Council to withdraw their funding contribution to this service is very disappointing.
“SWestrans and SPT will undertake a tendering exercise shortly for renewal of the contract.
“Whilst we are working to achieve the best outcome, the loss of 33 per cent of the funding is likely to have implications for the level of service which can be provided.”
Scottish Borders Council’s executive member for roads and infrastructure Gordon Edgar said: “The council is carrying out a subsidised bus services review which is aiming to make required changes – with budgets tighter than ever – while minimising the impact on bus passengers.
“As part of this review, the council intends not to continue with its current contribution to the 101/102 service as of August 2018, which stands at £135,000 per year.
“The council is considering its options, one of which is to extend the current 93 service from West Linton to Penicuik and create a connection with the X62 service which runs to and from Edinburgh.
“This would ensure travel opportunities are maintained for communities while providing considerable savings for the council.”