The father of a 6-year-old boy killed in the mass shooting in Newtown, Conn., wept Wednesday as he showed pictures of his son to senators and urged them to defy the political odds by implementing a new ban on military-style assault weapons.

Neil Heslin recounted that when he dropped off his son, Jesse Lewis, at Sandy Hook Elementary School on Dec. 14, the boy told him, “It’s going to be okay.”

“He told me it’d be okay,” Heslin said. “And it wasn’t okay. . . . I saw something that shouldn’t happen at an elementary school.”

Members of the audience wept as Heslin spoke. The emotional testimony came during a lengthy Senate Judiciary Committee hearing on the proposed assault-weapons ban, one of four Democratic-sponsored bills that the panel plans to begin debating Thursday.

Other proposals include a plan to expand the nation’s gun background-check system, make gun trafficking a federal crime for the first time and provide more federal funding to school districts to revamp school security plans. Republicans on the committee say they will exercise their right to delay consideration of the bills for another week.

The assault-weapons ban, introduced last month by Sen. Dianne Feinstein (D-Calif.), would prohibit almost 160 specific military-style firearms. The list includes semiautomatic rifles or pistols that can be used with detachable or fixed ammunition magazines that hold more than 10 rounds and have specific military-style features, such as pistol grips, grenade launchers or rocket launchers. Unlike a similar but narrower law that expired in 2004, the new ban would be permanent. Feinstein’s bill has 21 co-sponsors, all of them Democrats.

Feinstein used video examples during the hearing to demonstrate how easily shooters can fire rounds from some of the weapons she hopes to ban. “The need for a federal ban has never been greater,” she said, citing law enforcement officials and state governments who are pushing Congress to strengthen gun laws.

A majority of Americans support banning assault weapons, according to a recent Washington Post-ABC News poll. But most Senate Republicans and a mix of moderate Senate Democrats — many of whom face reelection next year — believe that any prohibition on specific rifles and shotguns is a politically untenable position in a country that still broadly supports Second Amendment rights. That makes passage in the full Senate all but impossible.

Judiciary Committee Chairman Patrick J. Leahy (D-Vt.) granted Feinstein’s request for a hearing as a courtesy, according to aides. Senators heard from eight witnesses, including John Walsh, the U.S. attorney for Colorado, and Edward Flynn, Milwaukee police chief.

Tempers flared when Sen. Lindsey O. Graham (R-S.C.) quizzed Walsh and Flynn on prosecutions of people who fail gun background checks.

“I want to stop 75,000 people from buying guns illegally,” Flynn told Graham, his voice rising. “That’s what a background check is.” When Graham asked Flynn how many background-check cases he had referred for prosecution, the chief snapped back: “We don’t chase paper. We chase armed criminals.”

William Begg, an emergency-room doctor who treated injured survivors of the Sandy Hook shooting, told lawmakers that the victims were hit by up to 11 bullets each, citing the state coroner’s report. “When a child has three to 11 bullets and it’s an assault-type bullet, it explodes in the body,” he said. “. . . That’s not a survivable injury.”

Sen. Charles E. Grassley (R-Iowa), the committee’s ranking Republican, said the previous ban did little to deter mass shooters or reduce gun-related violence.

“When something has been tried and not found to work, we should try different approaches rather than reenacting that which failed,” Grassley said.

He suggested that the committee is more likely to advance the other bills up for consideration and suggested finding ways to bolster the government’s funding for mental-health research.

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