WASHINGTON — President Barack Obama unveiled an array of measures on Tuesday tightening control and enforcement of firearms in the U.S., using his presidential powers in the absence of legal changes he implored Congress to pass.

Obama accused the gun lobby of taking Congress hostage, but said “they cannot hold America hostage.” He insisted it was possible to uphold the Second Amendment while doing something to tackle the frequency of mass shootings in the U.S. that he said had become “the new normal.”

Obama wiped tears from his cheeks as he spoke emotionally about the victims of gun violence.

“This is not a plot to take away everybody’s guns,” Obama said in a ceremony in the East Room. “You pass a background check, you purchase a firearm. The problem is some gun sellers have been operating under a different set of rules.”

At the centerpiece of Obama’s plan is a more sweeping definition of gun dealers that the administration hopes will expand the number of sales subject to background checks. Under current law, only federally licensed gun dealers must conduct background checks on buyers. But at gun shows, websites and flea markets, sellers often skirt that requirement by declining to register as licensed dealers. So new federal guidance from the Obama administration clarified that it applies to anyone “in the business” of selling firearms.

The White House also put sellers on notice that the administration planned to strengthen enforcement — including deploying 230 new examiners the FBI will hire to process background checks.

To lend a personal face to the issue, the White House assembled a cross-section of Americans whose lives were altered by the nation’s most searing recent gun tragedies, including former Rep. Gabrielle Giffords and relatives of victims from Charleston, S.C., at Virginia Tech. Mark Barden, whose son was shot to death at Sandy Hook Elementary School, introduced the president with a declaration that “we are better than this.”

Invoking the words of Martin Luther King Jr., Obama said, “We need to feel the fierce urgency of now.”