The Conservatives say they are adopting a "fresh approach" to politics with a pre-conference advertising campaign that will run exclusively online.
The Tories say people want a new approach to politics
The "It's Time for Change" adverts - nine pledges linking to the party's website - are running on Facebook and newspaper websites.
They say it is a "political first", but the Lib Dems say they have already run several exclusively online campaigns.
Labour says it has run several online ads as part of wider campaigns.
The adverts feature a picture of party leader David Cameron alongside pledges on subjects such as pensions, education and taxes - the party hopes they will be seen by about 21m people.
Party chairman Caroline Spelman said the "totally different" approach to campaigns demonstrated Mr Cameron's "fresh approach to politics".
"It's a political first, the first time a political party has had an entirely online advertising campaign, because we recognise that increasing numbers of people like to get their information that way," she told the BBC.
A party spokesman said it was "broader" than the party's previous "tosser" campaign - which was criticised by some for its language - which had its own website.
But the Liberal Democrats say they have already run several campaigns exclusively online - including a petition for the 91 Iraqi interpreters seeking asylum, which also featured banner adverts on the Times website.
Lib Dem elections co-ordinator Steve Webb said: "We all run campaigning websites and we haven't all gone shouting about it like this."
He added: "The whole thing strikes me as a bit of a yawn."
The party's head of innovations, Mark Pack, told the BBC the Lib Dems differed from the other two main parties, as they did "relatively little" advertising - but had embraced online ads because they tended to be "self financing" and reached specific target audiences.
He said he was surprised at the Tories' claim to be running the first exclusively online advertising campaign.
The Conservatives respond that it is the first banner ad campaign to put out their general message, rather than a specific campaign.
Labour blogger Tom Watson MP dismissed it as a "glossy advertising campaign" and said David Cameron should get his policies right first.
Labour uses internet advertising, but says it is "far more effective" to run it as part of a wider campaign, using other media as well.
A spokesman said: "In the Scottish elections for example we had a comprehensive online advertising campaign and that involved all the papers up there."
"This is absolutely nothing new."
Stephen Coleman, professor of political communication at Leeds University, said online advertising had proved very popular in the US, where it was possible to tailor it much more specifically to each person - something that data protection laws prevent in the UK.
He added that evidence from e-commerce studies suggested that online advertising would confirm people in their allegiances, but was unlikely to turn the heads of Facebook users who were not interested in politics.