Foreign and Commonwealth affairs spokesman Mr Umunna said his party's leader Jo Swinson is a "big wild card" and people "massively underestimate the impact that Jo will have during a general election campaign".
He spoke to the media following his conference speech in Bournemouth.
• READ MORE: Jo Swinson: Lib Dems are open to pro-remain pacts to stop BrexitMr Umunna, once heavily tipped for Labour leader, said: "I would hope that we get more than 40 seats at a general election ... but we know from the internal polling that if we move from the position that we're in and say there is a 1.5% to 2% swing we can get up to 100 seats and if there's a 5% swing towards the Liberal Democrats through the course of the campaign 200 seats are in contention - but who knows what will happen?
"We are not complacent, we are not drunk on our success, we want to continue to expand the bandwidth, draw people into the party."
The Streatham MP defected to the anti-Brexit party in June, having walked out of Labour with six other colleagues in February to stand as independents.
Ms Swinson received a huge boost from members over the weekend when they overwhelmingly backed her plan to revoke Article 50, should she be installed in Downing Street.
Defending the party's stance from claims it was undemocratic and overruled the Brexit referendum result of 2016, he said the revocation policy "will be anchored in a manifesto".
• READ MORE: 'There is no anti-SNP pact' Scottish Lib Dems deny new deal with the ToriesHe said: "The undemocratic thing would be not to implement your manifesto commitments and we are absolutely clear that we want to stop Brexit and the way we will do that in the manifesto is that we will revoke Article 50.
"Now if you get a Liberal Democrat majority Government, why should we not implement our manifesto?"
Mr Umunna said with hindsight he should have joined the Lib Dems - or rejoined as he was a member briefly as a teenager - a lot sooner.
He said it was not a "sustainable state of affairs" to go into Christmas with a Government that had a majority of minus 43 and a general election was "highly likely", adding: "Actually, the national interest dictates we've got to make sure that we secure the extension to Article 50 first."
Of the current Tory party, he said: "With the ejection of the One Nation Disraeli tradition from the Conservative Party, with the expulsion of the 21 MPs, one of whom is now a Liberal Democrat MP, you see the Conservative Party being reconfigured and reborn as a right-wing nationalist party."
During his conference speech, Mr Umunna turned his fire on Prime Minister Boris Johnson and his former leader Jeremy Corbyn.
He told delegates: "The Tory right who've taken over that party, they like nothing more than to bang on about incarcerating more and more people who break the law but strangely they think there should be a different approach to their law breaking."
He added: "He (Mr Johnson) likes to think of himself as a modern Winston Churchill ... he's been busy kicking out Winston Churchill's relatives from the Tory party."
Mr Umunna accused Mr Corbyn of acting as an "apologist for a hard-right Russian government".
He said: "You (Labour) cannot be a champion of liberalism if you are currently subject to a formal investigation by the Equality and Human Rights Commission for institutional racism against Jewish people.
"You cannot be a champion of liberalism when your leader's supporters think it is acceptable to abuse, vilify and deselect anyone who dares to question the leader.
"And you cannot claim to be liberal when the political editor of the BBC needs to take a bodyguard to your conference."
The Labour Party hit out and accused Mr Umunna of standing only for "own self-interest".
"This is more rank hypocrisy from someone who once called young activists 'dogs'," a Labour source said.
"He is now in the party that inflicted devastating austerity that has blighted our communities and he says Tory MPs who opposed LGBT+ rights share his values. The only value Chuka stands for is his own self interest."