President Obama and Vice President Biden formally announced the specifics of their gun-control proposals in a press conference Wednesday. (Nicki Demarco/The Washington Post)

In making his case Wednesday for tighter controls on gun ownership, President Obama turned to the document most often cited by firearms advocates in defense of gun rights — the Constitution.

By doing so, Obama sought to turn a perceived political weakness — his image as an aloof intellectual — into a strength, and, at the same time, to turn a perceived strength of gun advocates — the constitutional right to bear arms — into a potential weakness.

Citing a series of mass shootings, Obama listed several amendments, as well as the defining phrase of the Declaration of Independence, to argue that the right to bear arms should not compromise other rights.

“We have the right to worship freely and safely — that right was denied to Sikhs in Oak Creek, Wisconsin,” Obama said at a midday event. “The right to assemble peacefully — that right was denied shoppers in Clackamas, Oregon, and moviegoers in Aurora, Colorado.”

Obama added that “that most fundamental set of rights to life, liberty and the pursuit of happiness” were “denied to college students at Virginia Tech and high school students at Columbine and elementary school students in Newtown, and kids on street corners in Chicago on too frequent a basis to tolerate.”

President Obama proposed expansive gun-control policies aimed at curbing gun violence. The Obama administration can implement about half of the proposals, but the others — arguably some of the more critical initiatives — will require congressional approval.

“All the families who never imagined they’d lose a loved one to a bullet, those rights are at stake,” he said. “We’re responsible.”

Obama, a former constitutional law lecturer at the University of Chicago, has at times been forced to defend the legality of his efforts, in particular the health-care law he secured nearly three years ago.

While his audience in the past was Congress or the Supreme Court, Obama aimed beyond the Beltway on Wednesday to try to assure Americans that his proposals on guns amount to a modest approach to a societal problem.

Polls show that a majority of the electorate shares his views. But he warned Wednesday that those who do must apply pressure on interest groups and members of Congress.

“The only way we can change is if the American people demand it,” Obama said before an audience that included victims of gun violence and their families, among them parents of children killed last month in Newtown, Conn.

He added that that “doesn’t just mean from certain parts of the country” — the liberal states and enclaves where gun control has the most support.

“We’re going to need voices in those areas and those congressional districts where the tradition of gun ownership is strong to speak up and to say this is important,” he said.

His challenge became evident immediately, as a number of congressional Republicans from just the kind of conservative districts Obama referred to issued statements denouncing his plans.

Rep. Tim Huelskamp (Kan.) said in a statement that “the Second Amendment is non-negotiable.”

“The right to bear arms is a right, despite President Obama’s disdain for the Second Amendment and the Constitution’s limits on his power,” Huelskamp said. “Congress must stand firm for the entirety of the Constitution.”

Other Republicans followed suit, citing the Second Amendment in each case.

But Obama made clear Wednesday that he is eager to have an argument on constitutional grounds over whether a ban on the sale of assault weapons, new limits on high-capacity magazines and other elements of his gun-control package are legal.

At times in his first term, Obama has been criticized for sounding too much like a seminar leader at moments of ideologically charged and emotional debate.

His penchant for discursive explanations has bothered no constituency more than his base, whose members see in his sometimes professorial tone a lack of passion for the cause at hand.

At the start of his months-long pursuit of universal health care, Obama cited the need to “bend the cost curve” of entitlement spending as a chief rationale for the bill.

Obama’s liberal supporters, in particular, hoped for an argument based on the injustice that such a wealthy nation could have so many without access to health insurance. He made that point more often, and usually in appearances outside Washington, toward the end of the long effort.

But Obama has begun his push for a broad package of gun-control measures with more emotion, starting with the tears he shed publicly in the raw aftermath of the Newtown shooting, which left 20 children and six adults at Sandy Hook Elementary dead.

One student killed in Newtown was Grace McDonald, a 7-year-old who, Obama said Wednesday, “loved pink, and the beach, and dreamed of becoming a painter.”

He visited her family last month when he went to Newtown for a memorial service. As he left, Grace’s father, Chris, gave him one of her paintings, which hangs in his private study off the Oval Office.

“Every time I look at that painting, I think about Grace,” Obama said. “And I think about the life that she lived and the life that lay ahead of her, and most of all, I think about how, when it comes to protecting the most vulnerable among us, we must act now — for Grace.”