U.S. Democratic presidential candidate and U.S. Sen. Bernie Sanders speaks in Manchester, New Hampshire, Nov. 29, 2015. Mary Schwalm/Reuters

NEW YORK (Reuters) -- Democratic presidential candidate Bernie Sanders on Thursday urged the funding of gun violence studies at the U.S. Centers for Disease Control and Prevention, two decades after he voted against funding research into firearms injuries.

"We must authorize resources for the Centers for Disease Control and Prevention to study and research the causes and effects of gun violence in the United States of America," a Sanders campaign email said on Thursday.

The email came a day after 14 people were killed in a mass shooting in San Bernardino, California.

Congress, at the urging of gun rights supporters, put restrictions on CDC funding of gun research into the federal budget in 1996.

Sanders, then a Vermont U.S. representative, voted against an amendment, which ultimately failed, that would have authorized funding for such research, according to the website for the Office of the Clerk of the House of Representatives.

Sanders, now a senator, is vying with front-runner Hillary Clinton and former Maryland Governor Martin O'Malley for the Democratic nomination for the November 2016 presidential election. Gun control has emerged as an issue following a recent series of mass shootings.

Sanders has been dogged by criticism from gun-control groups since entering the presidential race. While in the House of Representatives, he supported a 2005 federal law that shielded gun manufacturers, distributors and dealers from civil liability for mass shootings, and voted against the 1993 Brady Bill that imposed mandatory background checks and waiting periods for gun purchases.

Residents of Vermont are generally protective of gun rights.

The Sanders campaign did not immediately respond to requests for comment on Thursday. But it has said he favors "sensible gun-control legislation" and that he supported Senate efforts to ban assault weapons and high-capacity magazines after the 2012 mass shooting at an elementary school in Connecticut.

(Reporting by Luciana Lopez; Editing by Peter Cooney)