U.S. Democratic presidential contenders on Saturday urged Congress to take action to curb gun violence following mass shootings last weekend in Texas and Ohio that left 31 dead.

Speaking at a hastily convened forum in Iowa, they called for the imposition of universal background checks on gun buyers, so-called "red flag" laws, and ultimately a ban on military-style assault weapons.

They also said they believed the long-standing debate on gun violence in America was shifting in favor of stronger restrictions.

"We are going to make change. We are going to pass gun safety laws in this country," said U.S. Senator Elizabeth Warren of Massachusetts.

The candidates took questions from gun-control advocates and shooting survivors at a program sponsored by Everytown for Gun Safety, an advocacy group founded by former New York Mayor Michael Bloomberg.

In the shootings in El Paso, Texas, and Dayton, Ohio, the gunmen used semi-automatic weapons with high-volume magazines.

Pete Buttigieg, the mayor of South Bend, Indiana, called for those weapons to be taken off the streets. 

"They have no basis in our neighborhoods in peacetime in the United States of America," Buttigieg said. 

Buttigieg was among a group of more than 200 mayors who sent a letter to Senate Majority Leader Mitch McConnell, a Republican, this week urging him to cancel the August recess and summon lawmakers back to Washington to vote on gun-control legislation. 

Several 2020 candidates highlighted how they would use the executive powers of the presidency to pursue gun-control measures if Congress again fails to act. 

Warren said she would impose increased background check requirements and more reporting on multiple gun purchases, and expand age restrictions to limit teenage access to guns. She said she would also push to do away with the filibuster, which would allow gun legislation to pass the Senate by a simple majority vote.

U.S. Senator Kamala Harris of California, who has said if elected she would give Congress 100 days to put a gun control bill on her desk, said they need to have "the courage to act."

Former Vice President Joe Biden, the current Democratic front-runner, touted his work in the Senate in the 1990s passing an assault weapons ban that has since expired. 

"I’ve taken on the NRA nationally, and I’ve beaten them," Biden said.

Many have called for measures such as an assault weapon ban, universal background checks and other gun control reforms long stymied by partisan fighting in Washington. 

Democrats have criticized Republican President Donald Trump’s mixed messaging this week on possible support for some gun control measures. 

Trump on Friday suggested that he could sway the nation’s powerful pro-gun lobby, the National Rifle Association, to drop its opposition to gun restrictions. He also said he had assured the organization that its viewpoint would be "fully represented."