Democratic front-runner Hillary Rodham Clinton is continuing a focus on gun control with a new television ad Tuesday that advocates stronger background checks for gun sales.

Gun control is perhaps the area of starkest policy difference to emerge so far between Clinton and her closest Democratic rival, Sen. Bernie Sanders (I-Vt.) -- a contrast which came into sharp relief during last month's first Democratic debate. The Clinton campaign has sought to highlight Sanders's mixed record on the issue, especially on background checks.

Clinton does not mention Sanders in the 30-second spot, which features an impassioned Clinton at a town hall-style forum, asking "how many people have to die" before background checks are "universal."

Sanders, who represents a state with a strong hunting tradition, voted against the Brady background check law and other measures. Background checks and waiting periods for buyers carry broad public support, something Clinton has pointed out frequently in the last month. She blames the National Rifle Association for blocking what she calls common-sense gun control, and urges hunters and responsible gun owners to buck the powerful lobby.

Clinton has proposed ways to narrow but not entirely close what is called the "gun-show loophole," in which buyers and sellers at gun shows and online gun markets can avoid background checks that apply at brick and mortar stores.

The ad will air in Iowa and New Hampshire as part of Clinton's already-extensive television campaign. She has had ads on the air since August and has spent more than $6 million on air time. Sanders put his first ad on the air this week.