People with learning disabilities are "marginalised" by the political process and parties should make it easier for them to vote, a report says.
Although 80% of those with learning disabilities were registered to vote in the 2005 election, only 16% did so, according to charity United Response.
It blames jargon-heavy manifestos and the complexity of the voting process for the low level of participation.
Details of candidates and policies should be more accessible, it argues.
It has set a target of 40% for turnout at the next general election, which must be held by June.
"People with learning disabilities are affected by decisions made at a national and local level in the same way as everyone else," says Su Sayer, United Response's chief executive.
"Yet information about the democratic process is often presented in a way which is confusing. As a result, many people who would like to vote currently find themselves excluded from a system which is meant to be there to represent them."
A new study of the issue, co-funded by the Electoral Commission, concludes that many of the one million people with learning disabilities in England and Wales who are entitled to vote are effectively disenfranchised by the political process.
It identifies low awareness of the right to vote as one of the "barriers" that needs to be overcome to increase participation.
It urges all parties to work together to engage more thoughtfully with those who feel excluded from political debate and for election candidates to communicate policies more clearly.
Its main recommendation is for parties to produce manifestos in an easier-to-read format and to make their election leaflets and websites more user-friendly.
Other ideas it believes are worth exploring to increase turnout include simplifying ballot papers and adding photos of candidates, as well as internet voting.
Looking further forward, it believes a new strategy is needed to boost democratic involvement, with all elected representatives taking more account of the needs of those with learning disabilities.