Rishi Sunak warned today that Brits might not 'ever' be able to get Labour out of power if they win the election
The PM delivered the stark message as he launched the Tories' Welsh manifesto, again accusing Keir Starmer of plotting tax hikes.
Mr Sunak conceded that voters were 'frustrated', but insisted he had 'heard' the criticism - and cautioned that sending the government a 'message' on polling day could be disastrous.
'I warn you, don't fall into Labour's trap, don't sleepwalk to July 4,' he said.
'I know you want to send us a message, but this is not a by-election. It will determine who governs our country for the next five years and potentially much longer.
Rishi Sunak delivered the stark message as he launched the Tories' Welsh manifesto, again accusing Keir Starmer of plotting tax hikes
The Tories launched their Welsh manifesto near Rhyl this morning
'For if Labour get in they will change the rules so it's much harder to ever get them out. They want to give 16-year-olds a vote not because on principle they think that they are adults, but because they think they'll vote for them.
'Once they have got power they will change every rule to make sure that they keep it. We can't let that happen, friends, and it's only we Conservatives who can stop it.'
Urging voters to give the Tories another term after 14 years in power, Mr Sunak said: 'Do not let Labour waltz into office without scrutinising them, without seeing what their policies would mean for our country and your family's security. Do you really know what you are going to get with Labour?'
Mr Sunak also warned that Sir Keir will look to expand a 'death tax' which is being considered in Labour-run Wales.
The premier sounded the alarm over the potential £295million raid on grieving families and also pledged to reverse 20mph zones imposed on drivers in the country.
The Labour-run Welsh administration is currently consulting on scrapping the council tax exemption for homes in probate, which would clobber grieving families with thousands of pounds in new taxes. The Tories say this would be a double whammy on top of inheritance tax.
Sir Keir has previously described Wales as his party's 'blueprint for what Labour can do across the UK'.
He has already pledged to follow the country's lead by allowing 16 and 17-year-olds to vote in elections, which the Tories claim is a bid to rig future polls because younger voters tend to vote Labour.
Mr Sunak warned that Sir Keir (pictured in Scotland today) will look to expand a 'death tax' which is being considered in Labour-run Wales
The Prime Minister was echoing a speech made by John Major ahead of the 1992 election in which the Tory premier warned that a vote for Labour could lead to the break-up of the UK because of the party's 'appeasement' of Scottish nationalism.
Sir John also warned that a vote for the pro-Brussels Labour or Liberal Democrats would see Britain join the Euro currency and thrust into a 'federal Europe' of closer integration.
The warning, four days before voting day, helped Sir John go on to win a fourth term for the Tories after 13 years in power despite opinion polls suggesting that Labour was ahead.