Rick Santorum announced the suspension of his campaign for the GOP presidential nomination on Tuesday, adding that he will not stop fighting for the defeat of incumbent U.S. President Barack Obama.

Speaking at his home state of Pennsylvania, Santorum said: "While this presidential race for us is over for me, and we will suspend our campaign effective today, we are not done fighting,"

"We are going to continue to fight for those voices, we are going to continue to fight for those Americans who stood up and gave us that air under our wings that allowed us to accomplish things that no political expert would ever have expected," he said.

Speaking in Gettysburg, near the famed Civil War battle site, the former Pennsylvania Senator said that United States "will never be a country that can go forward as a great and powerful country again unless we remember who we are and what makes us Americans. That's what our campaign was about."

Earlier in the day his campaign spokesman said the former Pennsylvania senator was due to make the announcement , two weeks before the GOP presidential primary there. Santorum faced a tough fight in his home state against Romney.

In respone to Santorum's annoucement, Romney released a statement, calling his former rival "an able and worthy competitor, and I congratulate him on the campaign he ran."

"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," Romney added.

Jim Messina, Obama's campaign manager, also responded to the news of Santorum's stepping down, saying that it was "no surprise that Mitt Romney finally was able to grind down his opponents under an avalanche of negative ads."

"But neither he nor his special interest allies will be able to buy the presidency with their negative attacks. The more the American people see of Mitt Romney, the less they like him and the less they trust him," Messina said, adding that while "calling himself the ‘ideal candidate’ for the Tea Party, he has promised to return to the same policies that created the economic crisis and has alienated women, middle class families, and Hispanic Americans."

"Americans value a President who will fight every day to rebuild an economy in which hard work will pay, responsibility is rewarded and everyone plays by the same rules. And that President is Barack Obama,” the president's campaign manager added.

Despite the fact that, technically, Santorum, by can continue his fund-raising activities, his suspension spells the effective end of his presidential race, one in which je began as the ultimate underdog, who saw some surprising ups and downs.

Santorum's withdrawal comes a little lest than a week after the current Republican frontrunner Mitt Romney made a leap toward winning the Republican presidential nomination with a clean sweep of Wisconsin and two other primaries, handing the former Pennsylvania senator a significant defeat.

The defeats for the conservative Santorum ramps up pressure on him to pull out of his increasingly bitter standoff with the former Massachusetts governor Romney.

In his victory speech in Milwaukee, Romney took aim at Obama over his handling of the U.S. economy and high gasoline prices.

"It's enough to make you think that years of flying around on Air Force One, surrounded by an adoring staff of true believers telling you what a great job you are doing, well, that might be enough to make you a little out of touch," Romney will say, according to speech excerpts released by his campaign.

Obama mentioned Romney's name in a speech on Tuesday, signaling that the White House now sees the millionaire former executive as effectively the Republican nominee.

He mocked Romney for backing a Republican budget plan authored by a key ally backer in Wisconsin, Congressman Paul Ryan.

Obama's sharp partisan tone, and Romney's swift and withering response, foreshadowed the campaign battles to come.
"He said that he's very supportive of this new budget and he even called it 'marvelous,' which is a word you don't often hear when it comes to describing a budget," Obama said.

Romney came from behind in the last couple of weeks to surge to the top of the polls in Wisconsin, and the loss further diminished former U.S. senator Santorum's hopes of a miracle finish to wrestle the nomination from his more moderate opponent.

Romney is now likely to have well over half of the 1,144 delegated needed to clinch the nomination at the Republican convention in August.