Democratic presidential hopeful California Senator Kamala Harris speaks during the third Democratic primary debate of the 2020 presidential campaign season hosted by ABC News in partnership with Univision at Texas Southern University in Houston, Texas on September 12, 2019.

Sen. Kamala Harris slammed President Donald Trump during the Democratic debate Thursday when the conversation turned to gun control and recent mass shootings.

"He didn't pull the trigger, but he certainly has been tweeting out the ammunition," Harris said.

The White House did not respond to CNBC's request for comment on Harris' comment about Trump.

The president's critics have slammed him in the wake of the shootings, drawing parallels between the president's rhetoric and the extremist opinions and motives of some recent mass shooters.

Trump has rejected such criticism. The president has also suggested the he could back stricter background checks, but has more often emphasized a desire to key in on mental health when it comes to preventing gun violence.

The National Rifle Association, the nation's most powerful firearms lobby, is opposed to background checks, and has pressured Trump against supporting them. The NRA spent $30 million to support Trump during the 2016 campaign.

Gun control has become an increasingly urgent issue among the Democratic presidential primary field as Washington watches for whether Trump will support legislation addressing background checks.

The gun issue took on particularly heavy weight during Thursday's debate, which occurred in Houston, Texas. In early August, twenty people died and 26 were injured at a shopping center in El, Paso, Texas, in what was "one of the deadliest days in the history of Texas," according to Texas Gov. Greg Abbott, a Republican. Shortly after, a shooting in Odessa, Texas, resulted in 7 deaths and 22 injured.

House Democrats have increasingly pressured the Republican Senate to take action on gun violence. On Thursday, leaders of 145 companies wrote a letter to the Senate, asking for it to pass background checks and a strong red flag law on Thursday. Senate Majority Leader Mitch McConnell, a Republican from Kentucky, has said he would not bring any gun legislation to the floor without Trump's support.

Former Rep. Beto O'Rourke, who returned from the campaign trail to his hometown El Paso shortly after the shootings, was praised by Harris and other candidates for his response.

When asked about mandatory gun buybacks, O'Rourke said, "If it's a weapon that was designed to kill people on a battlefield. " He added, "Hell yes, we're gonna take your AR-15s, your AK-47s."