The Tories have indicated they would protect schools funding in England after the general election.
But the Lib Dems say other aspects of education spending would be vulnerable under a Conservative government, which would have an impact on Welsh funding.
The Tories said the attack from their coalition partners was "desperate".
Meanwhile, Labour has defended its record on education in Wales after it came under attack from the Welsh secretary.
On Sunday, Tory Education Secretary Nicky Morgan appeared to suggest her party would ring-fence the schools budget for pupils in England aged five to 16.
Nick Clegg has said the Lib Dems would extend that protection on spending to cover the ages of two to 19, costing an extra Â£10bn a year.
Decisions about how much to spend on education in England have an impact on the Welsh government's budget because of the way changes to it are calculated through the Barnett formula.
Lib Dem MP for Cardiff Central Jenny Willott said: "The Liberal Democrats are the only party who have said we're committed to protecting cradle to college spending because we believe that best way to build a stronger economy and fairer society is to give every child the best possible start in life."
A Welsh Conservative spokesman said: "This is desperate stuff from a Liberal Democrat party that has seen its lowest poll rating over the weekend.
"It may have escaped Jenny Willot's attention that education in Wales is devolved.
"Welsh Conservatives want to put excellence back in our school system and cut red tape - ensuring more money goes to the frontline and teachers and parents decide what's best for pupils."
Welsh Education Minister Huw Lewis said that Conservative plans for education in England's schools were "a poxy programme for reform" and a "a desperately lame response to the challenges of the future economy".
Plans for education spending in England by the Conservatives could mean bigger cuts for the Welsh government, the Liberal Democrats have claimed.