GOP presidential candidate Rick Santorum, after defying long odds to become a top contender for the nomination, said Tuesday he is suspending his campaign, in effect paving the way for front-runner Mitt Romney to challenge President Obama.

Santorum’s announcement came as a surprise, though his campaign faced an increasingly unlikely path to the nomination in recent weeks.

The former Pennsylvania senator also on Friday left the campaign trail for the second time to tend to his 3-year-old daughter, Isabella, because she was hospitalized with pneumonia. She has a life-threatening genetic disorder known as Trisomy 18.

Santorum, far behind in the delegate race, had been outspent at every turn by the well-funded and better-organized Romney campaign.

“We will suspend our campaign effective today (but) we are not done fighting,” Santorum said, surrounded by his wife, Karen, and their other children at the Gettysburg Hotel in Pennsylvania, his home state where he faced a make-or-break primary April 24.

Romney has spent $2.9 million just in Pennsylvania for TV ads.

The Santorum announcement solidifies what already essentially had become a two-man fight for the presidency in November, with Romney and Obama over the past couple of weeks refining their campaign rhetoric to attack and attempt to define each other.

The remaining GOP candidates – Newt Gingrich and Ron Paul – have vowed to stay in the race until the end despite having little money and fewer primary wins and delegates than even Santorum.

Romney has 661 delegates, compared to 285 for Santorum, 136 for Gingrich and 51 for Paul, on his way to getting the 1,144 by June to get the nomination, according to the Associated Press.

Since his surprising win in the Iowa caucuses that kicked off the primary season, Santorum marched across the country steadfastly arguing that conservative social issue were as important to voters as the economy or jobs.

That message resonated across the Deep South, where Santorum won in Alabama, Louisiana, Mississippi and Tennessee on his way to 11 first-place finishes.

He called the campaign Tuesday "as improbable as any race that you will ever see for president."

Along the way, Santorum continually argued that he was the true conservative, not Romney, though the two often spoke by phone when the polls closed, and they did so Tuesday, as well.

Santorum campaign manager Jon Brabender told reporters his candidate called Romney and that the former Massachusetts governor asked that they meet.

Brabender’s comment will likely lead to further speculation about who Romney will pick as a running mate, though the Romney campaign has said no solid plans have been made.

“Senator Santorum is an able and worthy competitor, and I congratulate him on the campaign he ran,” Romney said. “He has proven himself to be an important voice in our party and in the nation. We both recognize that what is most important is putting the failures of the last three years behind us and setting America back on the path to prosperity.”

Still, Santorum did not endorse Romney in departing -- or even name him.

The Romney campaign did not say how the candidate would play the remaining 19 primaries as he sets his focus on stopping Obama from winning a second term.

However, his team issued a statement critical of Obama between Santorum’s announcement and the president’s speech before South Florida college students, in which Obama supported increasing the federal income tax on Americans making $1 million or more annually.

Romney also vowed in a campaign stop in Delaware to repeal the president's new health care law.

The Obama campaign also wasted no time.

"It's no surprise that Mitt Romney finally was able to grind down his opponents under an avalanche of negative ads,” said Jim Messina, the president’s campaign manager. “But neither he nor his special interest allies will be able to buy the presidency with their negative attacks."

Gingrich, a former House speaker, also praised Santorum.

“Rick has waged a remarkable campaign," he said. "His success is a testament to his tenacity and the power of conservative principles."

Gingrich also reiterated he is committed to staying in the race to the party's nominating convention in August in Tampa.

Paul campaign spokesman Jesse Benton said: “Congratulations to Senator Santorum on running such a spirited campaign. Dr. Paul is now the last – and real – conservative alternative to Mitt Romney. We plan to continue running hard, secure delegates and press the fight for limited, constitutional government in Tampa.”

Early Tuesday morning Santorum announced he would return to the campaign with two events in Pennsylvania, before his campaign said later that the first event would instead be a news conference, resulting in a flurry of speculation that the run was over.

Santorum said the end of the campaign was a family decision made over the weekend that was much like the one that started the campaign.

“Karen and I and the kids sat at the kitchen table and talked about our hopes and fears and our concerns,” he said. "And we were very concerned about our role as being the best parents we possibly could to our children and making sure that they had a country that was where the American dream was still possible.”

Santorum also reflected on the some of the most personal and meaningful moments on the trial, including how his sweater vest became “the official wardrobe of the Santorum campaign” and how Americans sometimes “overlooked by society” came out to see him, including those with disabilities like his toddler.

One person held a sign that read “I’m for Belle’s dad,” he said.