WESTERVILLE, Ohio — Joe Biden became the invisible man during Tuesday's Democratic primary debate, fading into the background as other presidential hopefuls and moderators focused much of their time on new top-polling candidate Elizabeth Warren.

An hour-and-a-half into the debate, Biden had spoken for 8 1/2 minutes, trailing Warren's 12 minutes of speaking time. He spent some of that time addressing the business dealings of his son, Hunter, who served on the board of a Ukrainian gas company while he was vice president. Biden dominated combined speaking time in the June, July, and August rounds of debates.

Previous debates centered on the then-front-runner's policy positions, and rivals went on the attack against him. In the June debate round, California Sen. Kamala Harris dug into Biden for working with segregationist senators in the 1970s to oppose integration busing. In September, former Housing Secretary Julian Castro hit Biden for supporting Obama-era immigration policies.

On Tuesday, Warren was the new target, with many candidates attacking her for failing to directly answer whether her healthcare plan would raise taxes on middle-class Americans.