By Joseph Ax

NEW YORK, Jan 25 (Reuters) - U.S. Attorney General Eric Holder on Friday gave a forceful defense of the Obama administration's efforts to institute gun control measures following last month's mass school shooting in Newtown, Connecticut, saying that partisan gridlock must not prevent meaningful reforms.

"We are bound and determined to do this," he said in a speech at the District Attorneys Association of the State of New York's winter conference in Manhattan. "This is not a time for complacency."

Gun violence has dominated national policy debate since the mid-December shooting in which Adam Lanza, 20, shot dead his mother and then killed 20 first-graders and six adults at an elementary school.

Earlier this month, New York became the first state to enact tougher gun laws following the Dec. 14 shooting, limiting magazine capacity and extending its ban on assault weapons.

Holder worked closely with advocates, experts and policymakers to help Vice President Joe Biden come up with a set of legislative proposals that would limit high-capacity magazines, reinstate the expired ban on assault weapons and require universal background checks for all gun sales.

In addition to those bills, President Barack Obama signed a series of executive orders that bolster the system for background checks and lifted the ban on federal research on gun violence, among other changes.

Some gun-rights advocates have accused the administration of attacking the Second Amendment right to bear arms, a criticism Holder dismissed.

"All of the president's actions have been consistent with the historical use of executive power -- and none will impinge on the Second Amendment rights of responsible, law-abiding citizens and gun owners," he said.

The gun control measures face sharp opposition from the powerful National Rifle Association, and their fate in Congress remains uncertain.

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