Questions over private health insurance have provoked the first clash of the Democratic presidential debate.

WATCH: Democratic presidential candidates face off in first 2020 debate

Former Texas Rep. Beto O’Rourke was explaining Wednesday why he supported a plan known as “Medicare for America” and not “Medicare for All,” saying he wants people who are happy with their current health insurance plan to keep it.

When pressed if he wanted to fully replace private insurance, O’Rourke said: “No.” That prompted New York Mayor Bill de Blasio to jump in from across the stage, saying, “Private insurance is not working.”

O’Rourke started to respond that people unhappy could choose Medicare, but de Blasio shot back, “Why are you defending private insurance?”

Only Massachusetts Sen. Elizabeth Warren and New York City Mayor Bill de Blasio raised their hands when asked if they would abolish private health insurance entirely in favor of a government-run plan.

Warren went on to note that she supports a “Medicare for All” plan proposed by fellow hopeful Vermont Sen. Bernie Sanders.

WATCH: Warren on why she backs ‘Medicare-for-all’

The issue opened into a debate among many of the 10 candidates on stage on the first of two back-to-back debate nights. De Blasio later interrupted Rep. Beto O’Rourke’s answer on how he’d handle health insurance by declaring that the current system is “not working.”