Charles Rex Arbogast / AP

Robin Kelly celebrates her special primary election win for Illinois' 2nd Congressional District, once held by Jesse Jackson Jr., over Debbie Halvorson, and Anthony Beale Tuesday, Feb. 26, 2013, in Matteson, Ill.

Democratic congressional candidate Robin Kelly, who centered her campaign heavily on calls for tougher gun control laws, emerged from a crowded field on Tuesday to clinch her party’s nomination for the Illinois House seat vacated by Jesse Jackson Jr.  

“You sent a message that was heard around our state and around the nation,” Kelly said in her victory speech late Tuesday night. “A message that tells the NRA that their days of holding our country hostage are coming to an end."

Her speech was focused almost solely on gun control, the issue that came to define the race in the Chicago-area district, an area of the country that has recently been at the epicenter of gun violence. Kelly skated to an easy victory, earning well over 50 percent of the votes with none of her competitors earning anywhere near that amount of support.

The former Illinois state representative was aided greatly by New York City Mayor Michael Bloomberg’s super PAC, Independence USA, which endorsed Kelly and spent more than $2 million in the race.

The PAC focused on taking down opponents for supporting certain gun-rights policies, including chief rival Debbie Halvorson, a former member of Congress.

"In the race to replace Jesse Jackson, watch out for Debbie Halvorson. When she was in Congress before, Halvorson got an 'A' from the NRA," argued an Independence USA TV ad, adding: "Debbie Halvorson -- when it comes to preventing gun violence, she gets an 'F.'"

Bloomberg tweeted his congratulations, writing, "As Congress considers the President's gun package voters in IL have spoken: we need common sense gun legislation now."

Prominently featured on Kelly’s website is a list of her five-point plan to reduce gun deaths.

"In Congress, Kelly will keep taking on the NRA, fighting to ban assault weapons and outlaw high-capacity ammunition clips," said one of her TV ads.

Kelly will go on to face a Republican challenger in April, but is expected to win easily in the heavily Democratic district.

NBC's Mark Murray contributed to this report

Updated 2:59 p.m. ET — Rep. Jesse Jackson Jr., D-Ill., resigned from Congress on Wednesday following a prolonged treatment for mental health issues.

An aide to House Speaker John Boehner, R-Ohio, told NBC News that the speaker's office received a letter from the Illinois congressman this afternoon.

"During this journey, I have made my share of mistakes," Jackson wrote in his letter. "I am aware of the ongoing federal investigation into my activities, and I am doing my best to address the situation responsibly, cooperate with the investigators, and accept responsibility for my mistakes, for they are my mistakes and mine alone. None of us is immune from our share of shortcomings or human frailties and I pray that I will be remembered for what I did right."

The son of the Rev. Jesse Jackson, the civil rights leader, Jackson, Jr. had sought treatment for bipolar depression at the Mayo Clinic for much of the past summer and fall. His last vote in Congress was on June 10, and his mysterious disappearance from official duties prompted speculation about the reason for the Democrat's prolonged absence.

Jackson was first elected to Congress in 1995 in a Chicago-area district's special election, and had won re-election to eight full terms since then. He won re-election just 15 days ago by a 40-point margin in the heavily Democratic district. Jackson also survived a Democratic primary challenge this summer from former Rep. Debbie Halvorson. President Barack Obama and Chicago Mayor Rahm Emanuel (D) each endorsed Jackson in the primary.

Amid his battles against mental illness, Jackson has also been embroiled in ethics allegations, which prompted the congressman to reportedly hire an attorney in recent weeks.

In particular, Jackson's efforts to convince then-Illinois Gov. Rod Blagojevich to appoint him as Barack Obama's successor in the Senate have drawn scrutiny. Blagojevich, a Democrat, was subsequently convicted of having sought favors and donations in exchange for the appointment.