The Judicial Proceedings Committee deadlocked 5-5 over the ban, which means the bill will likely not get a full vote on the Senate floor. With no corresponding legislation in the House of Delegates, that means the issue is almost certainly dead for the year.
Two Baltimore County Democrats - Sens. Norman K. Stone and James Brochin - joined three Republicans on the committee to kill the bill.
"The only people we target in this bill are law-abiding citizens who use these guns for target shooting or competitions," Brochin said. "I wish gun control worked, but it doesn't make any sense."
The bill, which was endorsed by Gov. Martin O'Malley, would have banned the importation, sale or possession of 45 types of semi-automatic handguns and long guns in Maryland.
The federal government's assault weapons ban ended in 2004, but most of the guns covered by that legislation still face stiffer regulations than other firearms under Maryland law.
Attorney General Douglas F. Gansler and the Maryland State Police testified in support of the bill, but the state Fraternal Order of Police opposed it, saying the law would be difficult to enforce.
The push to enact stricter gun laws in Maryland got stronger after the Washington-area sniper shootings of 2002, but efforts failed under former Gov. Robert L. Ehrlich Jr., who opposed an assault weapons ban.
Supporters had hoped that O'Malley's backing would make the difference, but the result this year was the same.
O'Malley spokesman Rick Abbruzzese said the governor is disappointed, but he said he could not say whether the governor will pursue the issue in the future.