Democratic 2020 presidential hopefuls made proposals ranging from firearm ­licensing to reimposing an assault weapons ban at an Iowa gun safety forum Saturday, one week after mass shootings that thrust the US' firearm violence into the presidential race.

Following back-to-back massacres that left a total of 31 people dead in El Paso, Texas and Dayton, Ohio, the candidates spoke largely in harmony on the issue that has seen little action at the federal level despite huge tolls from gun use, including nearly 40,000 deaths in 2017.

Front-runner and former vice president Joe Biden and other candidates like senators Elizabeth Warren and Bernie Sanders converged on the daylong event in the state that votes first to determine which Democrat will challenge President Donald Trump in 2020. 

The event comes as Trump grapples with steps Congress might take to tackle gun violence, including expanding background checks to virtually all people buying weapons, including purchases at gun shows. 

But it also follows Trump's repeated assertions that mental health issues and hate - not guns - are the main drivers of the deadly violence, and his talks with leaders of the National Rifle Association (NRA), which is opposed to expanding background checks.

Democrats came to the forum armed with various plans aimed at ultimately reducing the carnage that is a uniquely American problem within world's leading economies.

Several demanded that the Senate vote on the House-cleared measure expanding background checks, as well as one that codifies so-called red flag laws which allow authorities to confiscate weapons from people believed to present risks to themselves or others.

As the Democratic candidates criss-crossed Iowa in recent days, several took up the cause of gun safety, advocating for dramatic steps like reimposing an assault weapons ban that became law 25 years ago, only to sunset a decade later.

"I did that in 1994," Biden told the crowd, referring to his vote to pass the ban while he was a US senator. "We can do that again."

Biden advocated increased use of biometrics, saying as president he would seek a law requiring manufacturers to begin using technology like fingerprint-activated smart guns. And he joined other candidates like senators Kamala Harris and Kirsten Gillibrand in demanding that a law immunizing manufacturers from civil lawsuits be rescinded.