Conservative leader David Cameron will not guarantee the Dartford Crossing will stay under government ownership if his party wins the election, he has told the BBC.
Labour has announced plans to sell it, while the Liberal Democrats have talked about borrowing against it.
The Dartford Tunnel and Queen Elizabeth II bridge cross the River Thames between Dartford and Thurrock, forming a link in the M25 motorway, carrying about 150,000 vehicles a day.
The crossing is renowned as a major traffic bottleneck, with many people blaming the toll stations.
Last month, drivers were encouraged by campaigners to honk their horns at the crossing in protest at government plans to sell it as part of an attempt to shore up public finances.
'Difficult financial situation'
During a visit to a sixth-form college in Grays at the weekend, Mr Cameron said he understood people's concerns about the on-going tolls - but could not promise to scrap them.
Mr Cameron said: "We will have to look at all of these things. We obviously face a very difficult financial situation. But I quite understand the local concern about this issue."
A spokesman for the Liberal Democrats said: "We understand people's concerns about toll charges and the sale of the Dartford Crossing.
"The crossing is one of the assets that we would consider borrowing against in order to provide seed funding for the UK Infrastructure Bank, which would keep it in public ownership."
When asked about the Dartford Crossinig Labour's Environment Secretary Hilary Benn, on a visit to the Norfolk Broads, he said that in order to deal with the deficit, there are "decisions that need to be taken".
He talked about Labour policy on raising funds to bring down the deficit, saying its "about raising money and sustaining the economic recovery".