Ross D. Franklin / AP

Rep. Gabrielle Giffords and Ron Barber celebrate early Tuesday.

Updated at 12:54 a.m. ET: Democrat Ron Barber has won a special House election in southern Arizona to finish the term of former Rep. Gabrielle Giffords, The Associated Press projected.

Giffords, who was critically injured in a Jan. 2011 event in her district, resigned earlier this year.

A former aide to Giffords, Barber, who was also injured in the attack outside a supermarket that left six dead and 12 others injured, faced a challenge from Jesse Kelly, a Republican who narrowly lost to Giffords in the 2010 midterm elections.

The campaign was imbued with emotions that still linger from the attack, which forced Giffords’s retirement – despite remarkable progress toward recovery – earlier this year. But the special election campaign has also assumed a degree of political significance, given each party’s aggressive efforts to win the seat.

Bruising battle
The race evolved into a bruising battle between Kelly and Barber, fueled by hundreds of thousands of dollars of outside spending in the campaign. The district will be redrawn for this fall’s election, slightly in Democrats’ favor. But Giffords first won accolades for her political resiliency in a district that Republicans have won in the previous three presidential elections.

Democrats focused their resources on painting Kelly as an extremist who would seek radical changes to Medicare and Social Security even well beyond what most House Republicans had voted for in Wisconsin Rep. Paul Ryan’s budgets the past two years.

House Majority PAC, a Democratic super PAC, released one particularly effective ad featuring video of Kelly talking about eliminating corporate taxes and calling Social Security and Medicare Ponzi schemes.

Giffords herself made several public appearances in the past week – along with her husband, former astronaut Mark Kelly – pushing for her onetime aide.

A loss for Democrats would have threatened another demoralizing loss, though, on the heels of Republicans Gov. Scott Walker’s survival of a recall campaign last week in Wisconsin.

Both sides will inevitably spin the results of Tuesday’s primary as a harbinger for their chances come November, though the ultimate takeaways from this race might actually be more limited.

Testing ground for attacks
Democrats’ path toward retaking the majority in the House never ran through this seat, which they had controlled since Giffords first won in 2008. Barber will have to stand for re-election in November, and Republicans – including Kelly – vow to contest that race, too. But the district will favor Democrats more slightly, and it’s not clear how inclined that national Republicans will feel to invest heavily in that campaign.

But the race against Kelly provided Democrats in Washington a testing ground for their attacks against Republicans this fall associated with Medicare and Social Security.

Additionally, Republicans gained traction versus Barber by trying to tie him to House Minority Leader Nancy Pelosi and President Obama.

Barber had hedged for a period on whether he would support Pelosi as Democrats’ leader as elected, and he similarly punted on a May question about whether he would even vote for Obama. (Barber later clarified that he intended to vote for the president in November.)

But the fact that a Democratic candidate would feel the need to distance himself from Obama in Arizona raises questions about the viability of the president’s effort to win that state this fall. The Obama campaign has included Arizona in several of its Electoral College roadmaps; the state is rated Lean Republican in NBC News’ battleground map.

The Associated Press contributed to this report.

On the House floor Wednesday morning, the day after her appearance at the president’s State of the Union address in the same chamber, Arizona Democratic Rep. Gabrielle Giffords formally offered her resignation to Speaker John Boehner.

Walking with a limp and guided by her friend, Democratic Florida Rep. Debbie Wasserman Schultz, Giffords made her way to the well at the front of the chamber. Other members of the Arizona delegation surrounded her as Republican Arizona Rep. Jeff Flake held her hand.

A crying Wasserman Schultz applauded the strength of her colleague. "I am so proud of my friend," she said, placing on her hand on Giffords' back and wiping back tears. "It will always be one of the great treasures of my life to have met Gabby Giffords and to have served with her in this body,” the Florida congresswoman added.

"Even though I know we won’t see each other every day," she concluded, "We will be friends for life." The two then embraced, as Wasserman Schultz began to read Giffords' resignation letter.

"Even as I have worked to regain my speech, thank you for your faith in my ability to be your voice," said Giffords in her note. She vowed to focus on her recovery and to return, one day, to a life of public service.

"Everyday, I am working hard. I will recover and will return, and we will work together again, for Arizona and for all Americans," she pledged.

Read her resignation letter here (.pdf)

Assisted by Wasserman Schultz, Giffords climbed up to the speaker's perch and delivered the letter herself. An emotional chamber gave her a standing ovation.

Earlier, House Minority Leader Nancy Pelosi said in tribute, "God gave her a very special mission. He gave it to Gabby Giffords because He knew she could carry that burden because he had blessed her with so many, many gifts and a very loving family to make her the person that she is.”

Saul Loeb / AP

President Barack Obama embraces retiring Rep. Gabrielle Giffords, D-Ariz., on Jan. 24, 2012.

Her husband, retired Navy Capt. Mark Kelly, watched Schultz read his wife’s resignation letter, his hands covering his face at moments. Her mother, Gloria, sat next to him in the gallery.

Before resigning, Giffords voted in favor of a bill which she introduced shortly before being shot last January. It passed unanimously, 408-0.

Giffords was shot in the head during a constituent event in January 2011.  Six people were killed and 13, including Giffords, were injured. 

Slideshow: Ariz. Rep. Gabrielle Giffords

Saul Loeb / EPA

A look at the Arizona lawmaker's rise to prominence — from high school to Capitol Hill.

Launch slideshow

Updated at 6:02 p.m. ET

Arizona Rep. Gabrielle Giffords (D) will resign from Congress this week, she announced in a video message posted Sunday. 

Giffords, the victim of a gunshot wound to the head in an attack a year ago in her Arizona district, cited her continued work toward recovery as a reason for stepping down from her seat. 

"I have more work to do on my recovery, so to do what is best for Arizona I will step down this week," she said in a video message posted to YouTube. "I will return and we will work together for Arizona and this great country."

According to a statement posted on her Congressional website, Giffords will attend Tuesday night's State of the Union address as one of her final acts as a member of Congress before submitting her resignation to Speaker John Boehner on Wednesday.  The statement also says Giffords plans to "finish" the Congress on your Corner event where the shooting happened before she leaves office. 

"Gabby Giffords embodies the very best of what public service should be.  She's universally admired for qualities that transcend party or ideology - a dedication to fairness, a willingness to listen to different ideas, and a tireless commitment to the work of perfecting our union.  That's why the people of Arizona chose Gabby - to speak and fight and stand up for them," President Obama said in a statement Sunday evening.  "Gabby's cheerful presence will be missed in Washington.  But she will remain an inspiration to all whose lives she touched - myself included.  And I'm confident that we haven't seen the last of this extraordinary American."

Giffords has enjoyed a remarkable recovery since being shot in that Jan. 8, 2011 incident that left six dead. 

Prior to that shooting, she had been considered a rising Democratic star, and had been considering a bid for Senate this fall. During the course of her recovery, she has been absent from Capitol Hill except for a surprise return to vote in August on an agreement to raise the nation's debt ceiling.

“I salute Congresswoman Giffords for her service, and for the courage and perseverance she has shown in the face of tragedy.  She will be missed,” House Speaker John Boehner said in a statement. 

House Minority Leader Nancy Pelosi reacted to the announcement in a statement saying Giffords "has been a true bright star - a dynamic and creative public servant.  Gabby's message of bipartisanship and civility is one that all in Washington and the nation should honor and emulate."  Pelosi continued, "I join all my colleagues in Congress in thanking Gabby for the honor of calling her colleague and wishing Gabby and Mark great success and happiness.  She will be missed in the House of Representatives, but her legacy in the Congress and her leadership for our nation will certainly continue."