The Deputy Prime Minister is a supporter of Liz Truss’s leadership rival Rishi Sunak.
Mr Raab, a supporter of Rishi Sunak for the Tory leadership, said Ms Truss is proposing “limited” tax cuts via her plan to scrap the national insurance hike, arguing it would “do little” for the most vulnerable.
He described the policy proposal as “bad politics” in remarks that continue the blue-on-blue attacks which have become a dominant feature of the race to replace Boris Johnson as the next prime minister.
Ms Truss’s campaign has been forced on the defensive in recent days after the Foreign Secretary suggested there would be no “handouts” if she won the leadership contest and that her priority was reducing the tax burden.
But her allies have insisted she is committed to helping families struggling with soaring bills and further direct support has not been ruled out.
Ms Truss’s plan to reverse the national insurance increase, which Mr Sunak brought in as chancellor to boost NHS and social care funding, has come under fire for not offering enough to help the most vulnerable.
Mr Sunak has also signalled his plan would be to extend support packages he introduced during his time in the Treasury in a bid to ease the cost of soaring energy bills.
With the Bank of England forecasting inflation is set to hit 13% – and average household energy bills predicted to reach almost £4,000 – the Conservative MP said he would seek to press Whitehall departments to make savings to help fund cost-of-living support for millions of people during an “extremely tough” winter.
His backer Mr Raab, writing in The Times, said: “As Conservative Party members decide which way to cast their vote over the coming weeks, I urge them to consider this point carefully.
“If we go to the country in September with an emergency budget that fails to measure up to the task in hand, voters will not forgive us as they see their living standards eroded and the financial security they cherish disappear before their eyes.
“Such a failure will read unmistakenly to the public like an electoral suicide note and, as sure as night follows day, see our great party cast into the impotent oblivion of opposition.
“That’s the prize and that’s the jeopardy.
“If we make the right choices this autumn, our Conservative government can show, once again, that we have what it takes to lead the country through difficult times to a better future.
“Equally, the wrong move could prove economically harmful, and politically fatal.
“A response to the challenges people are facing that stops at limited tax cuts, which do little for the most vulnerable, isn’t Conservative politics. It’s bad politics.
“It will open the door of No 10 to Sir Keir Starmer, backed by the Lib Dems and the SNP — putting Brexit and the Union at risk, and ending the opportunity to make the 2020s a decade of low taxes and high growth.”
Mr Sunak said he would aim to keep any one-off borrowing to an “absolute minimum” by seeking “efficiency savings” across Government departments.
His team said the approach would aim to replicate previous measures used to fund support for Ukraine.
This resulted in departments and devolved administrations being asked to find underspends from their capital budgets, which involves money spent on investment and things used to create future growth.
Elsewhere, Ms Truss used an interview in the Daily Express to hit back at suggestions from Mr Sunak that her “starry-eyed boosterism” will not help the country resolve the problems it faces.
She said: “I’m not making any comments about other candidates in the race but the fact is there are too many people in the establishment of this country who want to talk our country down.”
Ms Truss also suggested the Treasury failed to focus enough on growth during Mr Sunak’s time as chancellor.
She said: “Frankly, there’s been too much Treasury orthodoxy and whether it’s on making sure investment is fair across the country, we need to do more to level up that investment, whether it’s on economic growth, I don’t think the Treasury has been sufficiently focused on economic growth.”
The Truss campaign also said the leadership contender will make tackling anti-social behaviour a “key priority” if she becomes prime minister.
Ms Truss would deliver on the Conservative manifesto commitment to recruit 20,000 additional police officers, and would ensure they focus on frontline policing and not be “overly burdened by form filling”, her campaign said.
The two Tory leadership hopefuls will continue campaigning on Tuesday before a hustings session with party members in Darlington, a so-called “red wall” area turned blue under Mr Johnson’s leadership in 2019.