I'd say a People's Budget that passes the first time. The middle and lower-middle class electorate would be alienated by the immediate increase in taxation without any simultaneous benefit, and would probably vote Conservative in an 1910/11 election.
But how to get the insane Tory Lords caucus to wave it through? If we handwave, then I'd agree that a purely anti-tax platform would work. Were the Liberals shut out of Ireland IOTL? IIRC it was only IPP and Unionists in those 103 seats by that point. ITTL they certainly would be. Asquith hadn't yet alienated the left through suffrage and national security policy, so ironically this might mean the Liberals survive in the long run.
OTL's January manifesto was almost entirely about the Lords, with cursory talk of the nascent welfare state and Imperial Preference. Nothing about the taxes. Why not do a cost accounting and say "they will impose x amount of additional taxation on y individual"? That seems like a far better strategy to me.
So let's say Balfour's back in No 10 come January 1910 with a seat count in the 375-390 range. What's the agenda? Manifesto doesn't have anything concrete for either domestic or foreign policy besides Imperial Preference and abolishing the Poor Law.
Also, how long does Balfour stay on as PM? On one hand, he might not want to fight an election in his mid-70s, but on the other caucus is probably still divided between Chamberlain and Long unless a new star emerges ITTL.