The number of people who are forced to rent their homes from private landlords because they cannot afford to buy could determine the result of the next general election in dozens of key parliamentary seats, research reveals today.

Polling by ComRes for the campaigning organisation Generation Rent finds that 35% of people in the private rented sector describe themselves as floating voters who could cast their votes on the basis of the parties' housing manifestos.

The figures, combined with analysis by the same organisation of the numbers of private renters in key constituencies, shows there are 86 parliamentary seats in which the sitting MP has a majority of less than 35% of the expanding tenant population. Alex Hilton, director of Generation Rent (formerly the National Private Tenants Organisation), said private tenants should be a priority for all the parties as they could directly affect who will be prime minister next year: "After years of being ignored by Westminster, Britain's private renters will decide whether MPs will keep their jobs."

Generation Rent is campaigning to address problems that beset those in the sector, including high rents, poor conditions, short-term and insecure tenancies and mistreatment by landlords and letting agents. It argues that tenants should be given better protection, including minimum standards as a precondition of renting out properties, a national register of landlords, licensed letting agents and longer and more secure tenancies.

With pressure on housing stock growing, the number of private tenants has continued to rise while the number of homeowners has been falling. In 2012-13, the English Housing Survey counted 14,337,000 homeowners, down on 14,388,000 in 2011-12. There were 3,956,000 private tenants in 2012-13, an increase from 3,843,000 in 2011-12.

Another report to be published in the House of Commons on Wednesday by the Smith Institute – entitled Poverty in Suburbia – will show that between 2001 and 2011 there was a 68% rise in the number of people renting in the private sector in suburban areas, as poverty has spread out from city centres to regions traditionally associated with greater affluence. It argues that higher property costs in city centres and a lack of available housing is forcing more out to a life of poverty in suburbia.

The government insists it is acting on several fronts to help more people buy their homes, and to expand the availability of social and private rented housing. Last week ministers said more than 17,000 homes had been bought under its Help to Buy scheme, which enables buyers to get on the housing ladder with as little as a 5% deposit through a loan from the government.

The figures were released days after the chancellor, George Osborne, announced that the scheme was to be extended until 2020. It also hails public-private deals under its Build to Rent scheme, which it says will allow work to begin by next year on 10,000 new homes that will be available for private rent.

However the Generation Rent analysis found that two-thirds of private renters (67%) felt stuck in the rented sector because of the cost of buying, and more than half (52%) said the level of their rent was their biggest problem.

Although rents have been rising as demand for properties has grown, one in three tenants said they were living in properties with unacceptable dampness and a similar proportion said their landlord did not seem very interested in their living conditions.

The shadow housing minister, Emma Reynolds, said that Labour was planning several initiatives to help those in the sector achieve higher standards, more security and better value in the private rental sector.

"Families need stability to plan where they will send their kids to school and certainty to manage their household budgets," she said.

"That's why Labour is committed to reforming the private rented sector so that it works for Britain's families. With longer-term tenancies and predictable rents, the private rented sector will offer the affordable and stable homes that renters need.

"It is a national scandal that more than a third of those people renting from private landlords are living in non-decent homes. If a local authority knows that poor standards are a massive problem in their area, we want to ensure they have the proper powers to deal with them."

Housing minister, Kris Hopkins, said: "We are taking action against the small minority of rogue landlords and letting agents, but avoiding excessive red tape, which would force up rents and reduce choice for tenants."

The full list of 86 MPs under threat as a result of the high number of private renters will be available fromMonday at Generation Rent .