Nigel Farage said he would push the Tories to spend "substantially" more than 2% of GDP on the armed forces as he prepared to unveil a manifesto showing Ukip was "the new party of defence".
In a bid to bolster slowing poll momentum, the anti-European Union party leader said it was offering "serious, fully costed policies" aimed at voters who "believe" in Britain.
An in/out referendum on Britain's EU membership "as soon as possible", a five-year ban on unskilled migrants coming in to the UK and a points-based system for others are among other key pledges.
Nigel Farage the leader of the UK Independence Party
It came as The Times reported sources close to European Commission President Jean-Claude Juncker saying no EU treaty changes were envisaged until after the end of his term in 2019.
Ukip will seize on that as evidence the Tory leader would be unable to put serious reforms to the UK's relationship to a vote in 2017.
With the party keen to broaden its appeal after a slip in support after surging to victory in last year's European elections, the manifesto also offers boosts for first-time buyers, military veterans, small businesses and hospital patients.
As the party prepared to launch its manifesto, challengers to Farage's position should Ukip fail to perform on May 7th are putting their heads above the parapet.
Steven Woolfe, a Ukip MEP and parliamentary candidate for Stockport, has admitted he would like to lead the party should Farage step down. Piers Morgan, presenting ITV1's Good Morning Britain, asked: "Yes or no, if he wasn't leader, would you want to be leader?"
Pressed on the issue, Woolfe eventually said: "Yes, of course I would."
Farage - who famously dismissed his party's 2010 manifesto as "486 pages of drivel" - said he was confident in the professionalism of the new document, and said it showed there was "real change on the horizon" in British politics for the first time in a century as the party seeks to secure enough of a Commons presence to wield some influence in negotiations in the event of another hung parliament.
It falls short of matching a Conservative pledge to find the £8 billion extra a year the head of the NHS says is needed by 2020 - proposing instead an additional £12 billion over five years with £5.2 billion being added to social care budgets.
Tories said Ukip had spent proposed savings from leaving the EU, reducing funding to Scotland and scrapping the HS2 high-speed rail project several times over, leaving a £37 billion "black hole" in its spending plans.
But Farage said the established Westminster parties had "repeatedly and knowingly raised the expectations of the public, only to let us down, time and time again.
"You will find serious, fully-costed policies that reflect what Ukip is all about: believing in our country," said Farage - who has said bringing forward the Tories' proposed 2017 EU referendum would be a key demand in any post-election deal.
"If you believe that we are big enough to make our own laws, in our own parliament; if you believe we should have the sovereign right to control our own borders; if you believe that we should be fiscally responsible, and stop adding to our national debts and expecting our children and grandchildren to pay the bill, then we are the party for you," he said.
Writing in the Daily Telegraph, Farage said the Tories' failure to commit to the Nato target in its manifesto would "rattle the dwindling number of Conservative voters who still see Cameron's party as the party of security.
"People won't be fooled. They know that under Mr Cameron, defence of the realm has fallen by the wayside. It no longer seems to be the first duty of government, closely followed by the idea that you look after your own, and those that need the most support, first."
Ukip's commitment to the armed forces "starts with honouring our 2% Nato spending commitment, and exceeding it substantially for the remaining years of the parliament", he said.
"Our current party leaders have failed to honour the military covenant, and Ukip has serious, costed ideas to stop the rot in this area," he said - including guaranteed jobs in the police, prison, or border force for anyone leaving the forces after 12 years or more, and a dedicated military hospital.
Farage said the party had "drilled right down into the detail on this".
He wrote: "There are major technical issues with the American F-35B Lightning II aircraft set to deploy on our aircraft carriers by 2020, and Ukip recognises the need to revise the armed forces' terms of service, to ensure personnel on operational duty overseas do not pay income tax."
Ukip will also promise to cut taxes for small business and waive stamp duty on new homes worth up to £250,000 built on brownfield sites.
Deputy chairwoman Suzanne Evans said Ukip offered common sense policies and was the only party which could prove its policies were affordable.
She told the BBC Radio 4 Today programme the whole manifesto had been reviewed by the Centre for Economics and Business Research to check the spending plans, admitting some policies had to be changed as a result.
Evans said: "It's been a very interesting process. We have made sure every policy in our manifesto has been rigorously tested and of course fully costed.
"What I think is going to be interesting is today we are going to be the only political party that is not just releasing a manifesto but alongside our manifesto we are going to be releasing a 20-page document that shows our policies have been fully costed and independently verified by the respected economic think-tank the CEBR.
"For us this is a line in the sand: we think all political parties should follow from now on and I don't understand why the others haven't done the same thing. Maybe they are not as confident in their spending pledges as we are."
Evans said the manifesto would include an immigration limit of 50,000 skilled workers from around the world, adding she was "proud" of the policy. She said the cap could later be removed as immigration slows after a decade of much higher numbers.
Unskilled workers and students would be allowed into Britain on temporary work visas, Evans said. She said: "We said before we would set up a migration control commission, so in a sense this takes it out of the remit of politics and into what the country needs.
"If there is a case we need agricultural workers, then why on earth would we stop them? It doesn't make any sense - Ukip is the party of common sense."
Evans said scrapping HS2, leaving the EU, cutting international aid and reviewing the Barnett Formula would produce £32 billion for spending commitments.
Rejecting Tory criticism, the Shrewsbury and Atcham candidate said: "The Conservatives' own spending proposals in the manifesto were ludicrous. They were quite clearly relying on some kind of weird magic money tree.
"The CEBR has challenged us at every single turn, we have had to change our spending plans accordingly. People are sick to death of politicians making promises in manifestos that they cam't keep. We are certainly 100% accurate on our numbers."