David Cameron has warned that Britain has "100 days to secure our future" as the mastermind behind his general election victory said the EU referendum is hanging in the balance.
In a new campaign video, the Prime Minister repeated claims that leaving the EU would push up prices in the shops, put jobs at risks and make the country's streets more dangerous.
But electoral guru Sir Lynton Crosby cautioned that so-called Project Fear may not work for the Remain campaign because the public sees risks in staying in the EU as well as quitting.
He predicted that the result was likely to hinge on which side could motivate supporters to come out and vote on June 23.
An ORB poll for the Daily Telegraph has suggested that battle is currently deadlocked, with Remain on 47% and Leave on 48%. However, when likelihood to vote is taken into account, the Brexit camp would win by 52% to 44%.
Former business secretary Lord Mandelson will further ratchet up the tensions later, accusing London mayor Boris Johnson of threatening the "special relationship" with the US by telling Barack Obama not to intervene in the referendum fight.
"There's an increasing tendency from Brexiters to damn anyone who deigns to take the opposing argument as having no right to do so," the Labour peer will say. "Boris Johnson's attack on President Obama is a classic of the genre."
Brexit-backing Cabinet minister Chris Grayling waded into the row on Monday night by joining calls for Mr Obama to stay neutral.
The Commons Leader said: "I would discourage any foreign leader from entering the debate at the moment. This is a matter for the British people and it should remain so."
Meanwhile, the CBI has delivered a boost for Remain by promising to make the "economic case" for staying in the EU - although the organisation will stop short of officially taking sides in the ballot.
In the video for Stronger In, Mr Cameron urged the public to "play your part" in the referendum.
"In a hundred days you'll have your say - whether Britain remains in a reformed Europe enjoying all the benefits that brings, like lower prices in the shops, more jobs and safer streets," he said. "Or whether those benefits are put at risk by leaving.
"We have a hundred days left to secure our future. It is a decision that will affect your future, your family's future and the lives of everyone in our country."
Sir Lynton - who has also advised Mr Johnson - wrote in an article for the Telegraph: "Those voters who are undecided or likely to change their minds believe risks of both Leave and Remain to be real, and locked in deadlock.
"So how to increase the risk for one side or the other to get ahead? If I was running either campaign, I would look to public services. The truth is that it is only with a strong economy you can pay for a good NHS, better schools and more police. But it is also true that the more pressure you put on the NHS, schools and the emergency services through greater use, the more they will suffer."