Since the Sandy Hook massacre in December 2012, President Obama has been adamant that Americans must fix an "epidemic" of gun violence, but proposals to increase background checks and limit assault weapons have been thwarted by a deeply-divided Congress.
On Tuesday, the president is expected to announce plans for a final push on gun control, forcing the issue even higher on 2016 campaigners' list of priorities.
After meeting with Attorney General Loretta Lynch and Federal Bureau of Investigations Director James Comey on Monday, Mr. Obama is expected to use executive actions to toughen gun sales laws, pushing to define more gun sellers as licensed "dealers" who must perform background checks on potential customers.
Under current law, only sellers who report firearms sales as their main livelihood are required to apply for a license from the Bureau of Alcohol, Tobacco, Firearms, and Explosives (AFT). Critics say that leaves far too many loopholes for people to sell guns without performing a background check.
But the White House must choose how to encourage those reforms: through legally binding "executive orders," which may be overturned by a court, or future laws, and all but guarantee lawsuits from guns' rights advocates; or through a variety of "executive actions," which are more like recommendations.
Republicans on the campaign trail have harshly criticized the prospect of an executive order, calling it an abuse of power. New Jersey Governor and presidential contender Chris Christie lambasted the president as a "king" and a "dictator," and fellow candidate Jeb Bush said the reforms would "take rights away from law-abiding citizens."
Many have promised to undo any changes if elected, including Donald Trump, Sen. Ted Cruz of Texas, and Sen. Marco Rubio of Florida.
Meanwhile, Democratic candidates Hillary Clinton and Sen. Bernie Sanders rallied supporters around support for gun control, as well as the fear of an elected Republican undoing Obama's proposed reforms.
Mrs. Clinton, who has also proposed executive actions on the issue, told New Hampshire supporters that a Republican in the White House "would delight in the very first day, reversing executive orders that President Obama has made."
Earlier this week, Sen. Sanders told ABC that executive actions on gun control were the "right thing to do," although he regretted that Congress could not reach an agreement.
The Democrats' renewed campaign focus on gun control marks a shift from recent presidential elections, in part because of party members' divided views. Even after winning his first election, Obama avoided contentious gun issues until the Sandy Hook shooting in December 2012, which killed 20 elementary school children and six staff.
But efforts to toughen gun control were unsuccessful in Congress, where the proposal failed to get 60 votes in the Senate.
Since then, mass shootings from South Carolina, to Oregon, to California have toughened the president's resolve.
"We have become numb to this," Obama mourned in October, after 10 people were killed by a gunman at Umpqua Community College in Roseburg, Ore.:
What’s also routine is that somebody, somewhere will comment and say, 'Obama politicized this issue.' Well, this is something we should politicize. It is relevant to our common life together, to the body politic.... We are the only advanced country on Earth that sees these kinds of mass shootings every few months.
In the past, Obama has blamed politics for Congress' failure to pass gun safety measures. After the bid to toughen background checks failed after Sandy Hook, he told the press that politicians "worry that that vocal minority of gun owners would come after them in future elections."
Since then, the gun control movement has gained powerful supporters, including former New York City Mayor Michael Bloomberg, who founded nonprofit Everytown for Gun Safety to counter the influence of the National Rifle Association.
Now "there's more organization, there's more capacity, there's more money," Duke University Professor Kristin Goss, an expert on gun control advocacy, told the Associated Press. In the past, vocal gun rights supporters have proven more politically engaged than gun control advocates, keeping pressure on their representatives to oppose expanded legislation.
But candidates' willingness to fire up primary voters before the February 1 Iowa caucus may fade as the general election draws near, and the importance of highly engaged single-issue voters' views must be balanced with the general public's.
"Both [sides] are really going to want to talk about [it] for the next couple of months, but I don't know how much they're going to want to talk about it in the fall," Matthew Dowd, a former political adviser to President George W. Bush, told the AP.
More than 90 percent of Americans approve of background checks for all gun buyers, according to an October CBS/New York Times poll. However, when asked if they support "stricter gun laws," that number falls to 58 percent, according to 2015 AP-GfK polls.
This report contains material from the Associated Press.