Mr Kennedy says his party is out-punching the Tories
The Liberal Democrat leader was speaking as he launched a dossier claiming to show the party's brand of "constructive opposition" was paying dividends.
The Lib Dem leader argued that his party was already "packing more of a punch" than the Tories under Iain Duncan Smith.
"The point that we want to make here is should there be an established opposition when there are two established opposition parties," he told BBC Radio 4's Today programme.
"I think that we are the alternative party of opposition.
"We are packing more of a punch than Mr Duncan Smith's Conservative Party, we're being listened to more by the commentators and I think that all stacks up to a fundamental change in British politics."
Mr Kennedy listed a series of scenarios in which he believes the Lib Dems have offered constructive opposition to the New Labour government.
Lacking substance
At a news conference at Parliament his message was: "We have acted responsibly and patriotically, endorsing the government's response to the terrorist threat - but not uncritically.
"In contrast, the Conservatives have had little of note to say. A policy of standing 'shoulder to shoulder' in all circumstances does not make for substance."
He said that the government had now adopted the Lib Dem policy on Railtrack, had changed its stance on tuition fees for students and planned to get rid of the voucher system for asylum seekers.
It was not enough simply to offer constructive criticism of policy - the Lib Dems would offer properly costed alternatives.
Mr Kennedy insisted that his party should have the same opposition rights as the Tories.
Parity
"All we are arguing for is a degree more parity for us," he said.
The Lib Dem move comes after an allegedly leaked party memo expressed concern that the party should not be seen as New Labour's poodles.
It also called for Mr Duncan Smith to be dismissed as "useless".
"I am concerned to be seen as my own person which is the way I hope I was recognised during the election," Mr Kennedy said.