The Liberal Democrat frontbencher David Laws has admitted rebuffing attempts to persuade him to defect to the Conservatives.
Mr Laws, the party's spokesman on Children, Schools and Families, said he was wooed a year ago by George Osborne, the shadow Chancellor, but rejected the overture.
Mr Laws, a member of the of "Orange Book" group of Liberal Democrat modernisers, said: "A year ago, George Osborne kindly asked me to become a Conservative. One or two of my colleagues may have thought I already was. But I am a liberal and a progressive. And in 2007, I am both sad and angry to find that we continue to live in a country in which your life chances are still framed by background, rather than ability.
"For Liberal Democrats that is simply unacceptable. So the Liberal Democrat Party must become the education party, the opportunity party, for a coming liberal century of personal empowerment."
Mr Laws outlined plans for a new £1.5bn "pupil premium" for children from low- income households. He said that headteachers and governors would choose whether to use the money for smaller class sizes, after-school classes, or one-to-one tuition.
Mr Laws also outlined plans for a new independent education standards authority to replace the Qualifications and Curriculum Authority, the Government's exams watchdog.
Vince Cable, the party's deputy leader, suggested that disadvantaged parents could use the pupil premium to send their children to private schools.
He said: "Maybe we should take this one step further and say, why can't this premium be used by these kids and these families to go wherever they want. I mean, if they want to use it to go as part-payment to get into an independent school, well why not? We'll back them up."