Former Housing Secretary Julin Castro took full advantage of his moment on the national stage at the Democratic debate Thursday night to unload on former Vice President Joe Biden , accusing him of betraying the positive aspects of President Barack Obama s legacy and refusing to disown the negative parts.

His sharpest exchanges with Biden came up during discussion of two of the defining battles of the Obama presidency: those around health care and immigration policy.

Health care came first, with ABC News panel of moderators beginning the debate in Houston with a heated and prolonged conversation about the merits and potential pitfalls of transitioning to a Medicare for All, single-payer health care system.

Castro, who stops short of embracing a single-payer plan that forces people to drop their private insurance, nonetheless delivered a blistering attack against Biden for his health care plan. Castro simultaneously cast doubt on Bidens self-appointed status as heir to Obamas presidency, his commitment to universal health care coverage and his mental acuity.

Biden argued that his proposed public health insurance option that Americans can buy into would achieve the same goals as the Medicare for All system proposed by Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.) without forcing Americans to drop their private insurance.

If you lose the job from your ... employer, you automatically can buy into this, Biden declared. No pre-existing condition can stop you from buying in. You get covered, period.

But Castro noted that by making enrollment in this public program optional rather than automatic, Bidens plan fails to guarantee universal coverage. Its a feature of Bidens plan that he disputed in the last presidential debate, but as CNN noted in a fact check , his plan only claims it will guarantee coverage for 97% of the population. In a country of 327 million people, that leaves 10 million Americans without coverage.

Barack Obamas vision was not to leave 10 million people uncovered, Castro said. He wanted every single person in this country covered. My plan would do that. Your plan would not.