Expert witnesses from leading landscape and outdoor recreation bodies who are against the upgrade are due to address a public inquiry this week.
Dave Morris, a director of Ramblers' Association Scotland, said candidates for the Scottish parliamentary and council elections should pay close attention to the evidence. He added: "This is the week for our aspirant politicians to learn what a disaster the proposed monster pylon route would be for Scotland's world-famous outdoor environment.
"Politicians cannot ignore the scale of opposition - from the 17,000 people who objected to the planning application to the 300,000 members of the National Trust for Scotland."
The inquiry is hearing evidence on plans for the 137-mile overhead line to replace the existing electricity transmission system to take power from proposed renewable-energy developments.
The plans, by Scottish Hydro Electric Transmission (SHETL), a subsidiary of Scottish & Southern Energy, and SP Transmission, a subsidiary of ScottishPower, have also attracted objections from the five planning authorities involved - Highland, Perth and Kinross, Stirling and Falkirk councils, and the Cairngorms National Park Authority.
The proposed 400kV transmission line, costing 350 million, would replace the existing 132kV line. It would use about 600 pylons - some 200 fewer than the one it will replace - but they would be more than 20 metres higher, at up to 67m.
Today, the inquiry will hear from John Mayhew, the chairman of Scottish Environment LINK, and David Jarman, speaking on behalf of the Scottish Wild Land Group.