Democratic presidential front-runner Joe Biden clashed with progressive challengers Elizabeth Warren and Bernie Sanders on healthcare in a debate on Thursday, defending Obamacare and pushing them to be honest about the cost of their plans.

The top 10 Democratic candidates shared the debate stage for the first time in Houston, focussing more on their shared opposition to Republican President Donald Trump and paring back some of the bickering that marked the first two debates this summer.

But the Democrats were quick to leap into battle on healthcare, the issue that has ignited the most heated debates in the campaign for the party’s nomination to face Trump in the November 2020 election.

Biden, who served as vice president for eight years under Barack Obama, said he would build on his landmark 2010 Affordable Care Act, and accused Warren and Sanders of wanting to tear it down with Medicare for All, a proposed government-run healthcare program that would eliminate private insurance.