Washington (CNN) -- Rick Santorum suspended his campaign for the Republican presidential nomination Tuesday.
The development means that Mitt Romney is now the certain GOP nominee to take on President Barack Obama in November, as Santorum was his main challenger. While Romney still needs to win several hundred delegates to clinch the nomination, Santorum's departure from the race leaves his path unhindered.
Santorum had canceled two events earlier Tuesday while adding an afternoon event that turned out to be a news conference.
Hogan Gidley, the campaign's communications director, said the two morning events were canceled to allow Santorum and his wife, Karen, to "settle in at home" with 3-year-old daughter Isabella after her weekend hospitalization.
Known as Bella, the child was born with Trisomy 18, a serious chromosomal condition that interferes with development. Half of patients with the condition do not survive past the first week of life, according to the National Institutes of Health.
Campaign aides have acknowledged that Pennsylvania, the state he represented in Congress, is a must-win for his candidacy when it holds its primary on April 24.
Santorum's once double-digit lead there slipped to single digits in a recent poll, and the cancellation of campaign events Monday and again on Tuesday morning stoked media speculation that Santorum might drop out in the face of front-runner Romney's commanding lead.
Meanwhile, a new poll Tuesday shows Romney trailing Obama in a head-to-head matchup, though voters remain split on which presidential contender is best equipped to handle the economy.
The survey, from Washington Post/ABC News, showed 51% of Americans would choose Obama if the election were held now, compared to 44% for Romney.
According to the polling data, Americans are divided on which candidate would best handle economic issues: Forty-seven percent favored Romney while 43% named Obama. When asked which man would be better at creating jobs, 46% named Obama and 43% said Romney. Both margins were within the poll's 3.5% sampling error.
Voters were less divided on other key issues. Fifty-three percent said Obama was best poised to handle international affairs, compared to 36% who said Romney. Conversely, when asked which man would do a better job of reducing the federal deficit, 51% said Romney and 38% said Obama.
In terms of likability, Obama held a clear advantage, with 64% of Americans polled saying the president was a more friendly and likable person, a nearly 2-to-1 advantage over Romney, who was at 26%.
The serious gender gap between the two candidates -- also seen in recent Gallup and CNN/ORC polls -- also appeared in the new poll. Obama had the support of 57% of women, compared to 38% who said they backed Romney, while the former Massachusetts governor had the backing of 52% of men, compared to 44% who backed Obama.
Among another important voting block, independents, the poll shows a much tighter race, with 48% backing Romney compared to 46% for Obama, also within the survey's sampling error.
Romney has used a huge advantage in money and organization to build his lead over Santorum and fellow challengers Newt Gingrich and Texas Rep. Ron Paul. In particular, the Romney campaign and the super PAC supporting it have spent millions of dollars on negative ads in the run-up to major primary and caucus votes so far.
On Monday, though, Romney's campaign pulled a television ad hitting Santorum while the former senator from Pennsylvania tended to his daughter in the hospital.
The sharply negative ad, which was to begin airing in Pennsylvania, highlighted Santorum's 2006 re-election loss in the state.
Romney campaign spokeswoman Andrea Saul said the decision was made "out of deference to Sen. Santorum's decision to suspend his campaign for personal family reasons."
Despite Saul's use of the word "suspend," Santorum has not formally suspended his campaign.
Santorum is Romney's closest rival but still has less than half of his delegate total, with Gingrich and Paul trailing far behind. However, the primary races after April 24 include states with conservative and Christian evangelical voting blocs that have backed Santorum so far.
Both Santorum and Gingrich have resisted calls to end their races in the face of Romney's substantial lead.
However, Gingrich admitted Sunday that Romney was the likely nominee.
"I think you have to be realistic," Gingrich said on "Fox News Sunday." "Given the size of his organization, given the number of primaries he's won, he is far and away the most likely Republican nominee."
CNN's latest estimate of the GOP delegate tally shows Romney with 659, Santorum with 275, Gingrich with 140 and Paul with 71. It takes 1,144 delegates to clinch the nomination.
New York, Connecticut, Rhode Island and Delaware vote on April 24, in addition to Pennsylvania. In all, 231 delegates are up for grabs in the five states.
Gingrich has said he will stay in the race until the Republican National Convention in August, despite running a distant third in the delegate count. The goal for both Gingrich and Santorum at this point appears to be preventing Romney from reaching the 1,144-delegate threshold before the convention.
While all but conceding the GOP race, Gingrich said Sunday he won't give up on trying to influence the party's platform that emerges going into the general election.
"I think platforms matter in the long run in the evolution of the party," the former House speaker said. "And the party is more than just a presidential candidate -- it's Senate candidates, House candidates, state legislators."
Gingrich also said he has already talked to Republican National Committee Chairman Reince Priebus about working in the fall "to help defeat Obama any way I could -- whatever the team thinks I can do to be helpful, I would do."
Beyond that, he said he wouldn't want to serve in a Romney administration and would rather "go back to a post-political career."
CNN's Kevin Liptak, John King, Peter Hamby, Shannon Travis and Steve Brusk contributed to this report.