* Obama's plan faces stern challenge in Congress
* Former congresswoman Giffords to make appearance
* NRA executive LaPierre is among the witnesses
* Hearing set to begin at 10 a.m. EST/1500 GMT (Updates to include Giffords appearance)
By Thomas Ferraro
WASHINGTON, Jan 30 (Reuters) - Six weeks after the massacre of 20 children and six adults at a Connecticut school ignited new calls to curb U.S. gun violence, the issue reaches Congress on Wednesday amid questions about whether lawmakers can agree on significant legislation.
In hearings that begin in the Senate Judiciary Committee on Wednesday, President Barack Obama and other Democrats are seeking the largest gun-control package in decades.
Former Arizona congresswoman Gabby Giffords, grievously hurt in a 2011 mass shooting that left six people dead and 13 wounded, will make an appearance at the hearing.
According to a senior Democratic aide and a person familiar with the proceedings, Giffords will testify. Her husband, former astronaut Mark Kelly, also is due to testify. The couple recently founded Americans for Responsible Solutions, a group intended to combat gun violence.
Others set to testify include National Rifle Association Executive Vice President and CEO Wayne LaPierre, whose group is an influential opponent of gun restrictions.
Obama's proposals to curb gun violence include reinstating the U.S. ban on military-style "assault" weapons, limiting the capacity of ammunition magazines, and more extensive background checks of prospective gun buyers, largely to verify whether they have a history of crime or mental illness.
Republicans and some pro-gun Democrats envision a more modest package. It is unclear whether there is sufficient support in the Democrat-led Senate and the Republican-led House of Representatives to pass any gun restrictions beyond improved background checks.
"We are trying to weigh things that could make a big difference against things that can pass," said Democratic Senator Charles Schumer of New York. "I think background checks is the sweet spot."
That sentiment reflects how the calls for gun control - so prominent during the emotional days following the Dec. 14 shootings in Connecticut - will face political reality in Congress.
"We must come together today as Americans seeking common cause," Senate Judiciary Committee Chairman Patrick Leahy, a Vermont Democrat, said in his prepared opening statement for the hearing.
Facing opposition by the NRA and others who assert that new restrictions on firearms would violate their constitutional right to bear arms, Leahy, a gun owner, said: "Let us forego sloganeering, demagoguery and partisan recriminations."
"This is too important for all that. We all abhor the recent tragedies - in just the last two years - in an elementary school in Connecticut, in a movie theater in Colorado, in a sacred place of worship in Wisconsin, and in front of a shopping mall in Arizona."
Giffords, an Arizona Democrat who was the target of an assassination attempt, survived a head wound but later retired from the House.
Most Republicans and some Democrats in Congress favor gun rights and represent constituents who do as well. The NRA has called any attempt to restrict weapon sales an assault on Americans' constitutional right to bear arms.
In recent days, some Republican lawmakers have joined Schumer and other Democrats in emphasizing better background checks of gun buyers, rather than Obama's plan to ban the sale of rapid-firing assault weapons like the one used in the Connecticut shootings.
Republican Senator Jeff Flake of Arizona, a Judiciary Committee member, said that "we all recognize the need for more effective background checks." But, Flake said, "people say responsible gun owners should be able to own any type of weapon or (ammunition) clip within reason."
The NRA's plan for securing schools has revolved around putting armed guards on campuses. In a statement released Tuesday that he plans to give before the Senate panel on Wednesday, LaPierre sounded a familiar refrain of gun-rights supporters, calling on better enforcement of existing gun laws rather than new laws.
"We need to look at the full range of mental health issues, from early detection and treatment, to civil commitment laws, to privacy laws that needlessly prevent mental health records from being included in the National Instant Criminal Background Check System," he said.
Federally licensed firearms dealers are required to run background checks for criminal records on gun buyers. But the government estimates that 40 percent of purchasers avoid screening by getting their guns from private sellers, including those at gun shows.
The White House's plan would require screening for all prospective buyers.
The background check provision is generally regarded as the gun-control measure most likely to receive bipartisan support, but even it could face some difficulty.
Although Obama's Democrats hold a 55-45 edge in seats in the U.S. Senate, the president's call to revive the assault weapons ban that expired in 2004 faces an uphill fight. (Editing by David Lindsey and Will Dunham)