Mr Nesbitt said he was proud of his party colleagues for deciding to leave Stormont’s powersharing executive in the wake of May’s Assembly election, as did the SDLP, and opting to take up the new option of establishing an official opposition.
“Forming an opposition is entirely consistent with who we are - a party ready and willing to do what’s right for the country, whatever the consequences for ourselves,” he said.
“It has been a bold step, but a necessary one for those of us who seek normal, democratic politics in Northern Ireland.”
He insisted the UUP and SDLP, once the pre-eminent political forces within their respective traditions, could regenerate and pose a serious threat to the current Sinn Fein/Democratic Unionist administration.
Taking to the stage in the Ramada Hotel in Belfast shortly after SDLP leader Colum Eastwood addressed the conference in an historic first, the UUP leader portrayed their respective parties as a joint ticket that could deliver for the electorate in 2021.
“Vote me, you get Colum. Vote Colum, you get me,” he said.
“Vote Colum and me, and you get a whole new middle ground politics, dedicated to making Northern Ireland work, whatever our motivations.”
He added: “Do we agree on everything? Of course not.
“But can we find a way of doing business together? Absolutely.”
Formally welcoming Mr Eastwood to conference, Mr Nesbitt said people should not “hold their breath” for similar symbolic moves between the DUP and Sinn Fein.
During his speech, the UUP leader and former TV broadcaster acknowledged his disappointment that his party did not win more than the 16 seats it secured in the Assembly election.
But he urged those who doubted his leadership to hold course.
“You know I wanted better - and I know you did too,” he said.
“But we did nothing wrong.”
He claimed the “politics of fear” still held sway when it came to the election, accusing the DUP of being its main peddler.
“Like the SDLP, we tried for an Assembly election focused on bread and butter issues, but we all know from the doorstep, the fear factor remained dominant,” he said.
He added: “So, to anyone looking to press the panic button - to the very few who have jumped ship - to the occasional whisperer and malcontent - I say this. Are you so weak, you want to unravel four years’ work, just because we didn’t get all we wanted first time around? Are you so easily led that a siren voice in your ear is enough to make you change course?
“I’ll tell you what I did after the elections.
“I dusted myself off, gave myself a good boot up the backside and got on with it.
“Please, do not fall for propaganda, from political enemies who understand a united Ulster Unionist Party, outside the executive, free to criticise and offer alternatives, is a serious threat. Especially, an Ulster Unionist Party working well with the SDLP.”
Mr Nesbitt listed a series of issues on which he claimed the current executive had let people down.
On the issue of Brexit, he said: “No one doubts Northern Ireland will be the most affected nation or region of the United Kingdom by withdrawal from the European Union, yet extraordinarily, we are still the least prepared, because of the paralysis of our DUP/Sinn Fein government.”
Touching on the thorny issue of dealing with the legacy of the Troubles, Mr Nesbitt called on all those involved to make statements to acknowledge their role in the sectarian strife.
He said Northern Ireland needed “political courage” of the kind shown by the Queen when she visited the Irish Republic.
“If (Sinn Fein’s) Martin McGuinness was prepared to make such an acknowledgement statement, he might be surprised by the responses that could follow, from the British Government, and from Dublin and Washington, and from the local political parties,” he said.
“This is not about shallow apologies. It’s about acknowledging our past, our choices, and their impact, still felt today.
“I believe that would transform how we deal with the past.”