WASHINGTON (CNN) -- With polls showing support for new gun legislation on the wane, President Barack Obama joined police officials and victims of gun violence Thursday to raise pressure on Congress to get something passed more than three months after the Newtown school massacre.

The president called on Congress to pass a package of gun laws coming up in the Senate, saying "none of these ideas should be controversial."

He also noted that "some powerful voices on the other side" want to delay action on gun legislation.

"Their assumption is that people will just forget about it," the president said.

The White House event comes on what advocates of tougher gun laws call a national day of action, with rallies and other gatherings planned in cities across the country.

Last week, a group called Mayors Against Illegal Guns launched a $12 million ad campaign targeting members of Congress in 10 states to act on legislation proposed by Senate Democrats and backed by the president.

The package includes expanded background checks, tougher laws against gun trafficking and straw purchases, and efforts to improve school security.

However, fierce opposition by the National Rifle Association and conservatives in Congress appears to have already doomed another proposal that would ban semiautomatic firearms modeled after military assault weapons.

While Senate Majority Leader Harry Reid says the ban proposed by Sen. Dianne Feinstein, D-California, can still be offered as an amendment, he dropped it from the package going to the Senate floor because it lacks enough support to overcome a GOP filibuster.

Even if gun legislation passes the Democratic-led Senate, it has less chance of winning approval in the Republican-controlled House.

Obama and others pushing for tougher gun laws say the December attack by a lone gunman that killed 20 first-graders and six educators at Sandy Hook Elementary School in Newtown, Connecticut, showed the need for national action against gun violence.

They note the killer in Newtown used a semiautomatic rifle that would be banned under Feinstein's proposal. The ban also would limit ammunition magazines to 10 rounds.

Opponents of tougher gun laws, led by the NRA, argue most gun violence involves pistols in urban areas, rather than the semiautomatic firearms targeted by Feinstein. Better enforcement of existing laws and posting armed security guards in schools would be more effective remedies, according to the NRA.

Police released new documents Thursday related to the Newtown shootings that showed the attacker, 20-year-old Adam Lanza, had a gun safe in his bedroom. The documents said that more than 1,600 rounds of ammunition were found in the home where Lanza killed his mother with one of her guns, shooting her in the forehead as she lay in bed.

Lanza then went to the elementary school, shooting his way inside and opening fire on classrooms with a semiautomatic rifle before killing himself in the rampage that lasted five minutes, the documents showed.

"This is exactly why we need to ban high-capacity magazines and why we need to tighten our assault weapons ban," Connecticut Gov. Dannel Malloy said Thursday. "I don't know what more we can need to know before we take decisive action to prevent gun violence."

After the Newtown shootings, some states -- including New York -- have passed tougher gun laws.

Polls conducted over the past few weeks suggest that more than three months after the Connecticut killings, public backing for major new gun laws has dropped.

A CBS News survey released this week indicated a 10-point decrease in support of stricter gun laws, from 57% immediately after the Newtown shootings to 47% now.

That poll was in line with a CNN/ORC International survey released last week that indicated a nine-point drop in the percentage of Americans who favor major restrictions on guns or an outright ban on gun ownership, from 52% following the shootings to 43%.

Other polls have shown changes in the same downward direction.

"Opinion on gun control was fairly steady over the past few years, but seemed to spike after the Connecticut shootings," CNN Polling Director Keating Holland said. "The big question is whether support for major new gun laws has simply dropped back down to that previous level or whether the slide will continue even further."

He noted that the biggest drop came among two specific demographics -- older Americans and people who live in rural areas.

"In the immediate aftermath of the shootings in Connecticut, the number of rural Americans who supported major gun restrictions rose to 49% but now that support has dropped 22 points," Holland said. "Support for stricter gun laws dropped 16 points among Americans over 50 years old in that same time."

To Richard J. Davis, the assistant Treasury secretary for enforcement and operations during the Carter administration, any new gun legislation that lacks some kind of ban on assault-style weapons and restrictions on ammunition magazines would be sad but not shocking.

"A sensible approach to gun violence would, among other non-law enforcement steps, include prohibitions directed at assault-type weapons, more regulation of handguns, less regulation of traditional long guns and working to make sure that federal, state and local law enforcement have the tools to enforce the laws relating to firearms," Davis wrote in a CNN opinion piece. "Unfortunately, logic does not always operate when the topic is guns."

CNN's Paul Steinhauser, Susan Candiotti and Ben Brumfield contributed to this report.