After a dramatic night of suspense in Iowa caucuses, the big winner may well have been a Democrat: Barack Obama.
Romney, a former Massachusetts governor, emerges from Iowa with his front-runner status intact, his well-funded campaign ready for a months-long fight.
But his razor-thin margin over Santorum - a social conservative who ran a low-budget campaign with little advertising - reinforces persistent doubts about Romney`s ability to win over his party`s conservative base.
It also increases the chances that Romney`s still-likely march to the Republican nomination will not be the quick kill Romney has hoped for, analysts and strategists said on Wednesday.
For an Obama campaign that has long operated on the assumption that it will face Romney in the November 6 election, that is good news.
"Democratic heavyweights are quietly celebrating tonight," David Gergen, a former adviser to two Republican and two Democratic presidents, told Reuters. "They see the presumed (Republican) nominee, Mitt Romney, unable to close the deal and a Republican electorate not only uncertain, but lacking great enthusiasm."
As Romney continues to tussle with Republican foes in upcoming primaries, Gergen said, "Obama`s campaign - which otherwise might be in trouble" amid concerns about the economy and government spending - "has time to raise money and hone its message."
For Romney, the good news in Iowa was that the two candidates who seemed in best position to carry out a long campaign against him - Newt Gingrich and Rick Perry - did not get a boost from the caucuses.
Gingrich, a former House of Representatives speaker, finished fourth and leaves Iowa stewing over seeing his status atop public opinion polls toppled by biting TV ads put out by an independent group that supports Romney.
Starting with the New Hampshire primary on January 10 and continuing on to contests in South Carolina and Florida, Gingrich has vowed to cast Romney as too moderate for most Republican voters.
Romney is heavily favored to win in New Hampshire, but Gingrich has led recent polls in South Carolina and Florida.
Perry, the Texas governor, stumbled to a fifth-place finish in Iowa and is returning home to "assess" his campaign.
A key question now is whether Santorum, who has little national campaign structure or money after making Iowa the focus of his effort, can turn himself into a nationally appealing, anti-Romney alternative for conservative voters.
Santorum, who peppers his speeches with religious and anti-abortion references, will also have to prove that he can stretch his appeal beyond the most conservative elements of the Republican Party.
Showdown in South Carolina
The Iowa result sets up a fascinating showdown on January 21 in South Carolina, another state with a strong Republican conservative electorate.
There, Romney - presumably after winning New Hampshire - will face a feisty Gingrich, who is from neighboring Georgia.
Gingrich is buying ads that call Romney a "Massachusetts moderate." And in two debates in New Hampshire this weekend, Gingrich will have a chance to take aim at Romney over what Gingrich has characterized at untrue and unfair attacks by Romney supporters.
If Santorum can quickly put together an organization in South Carolina to build support among influential evangelical Christians, he also could be a factor there. His effort could be helped by the withdrawal of Perry, who like Santorum has targeted conservative Christian voters.
Otherwise Santorum could fade from contention, as was the case with former Arkansas Governor Mike Huckabee, the evangelical-backed winner in Iowa in 2008 who could not sustain the momentum from that victory.
Also likely to contend in South Carolina will be Texas Representative Ron Paul, a libertarian with intense support from younger voters and independents who ran a strong third in Iowa.
Paul is not widely viewed as a possible winner of the Republican nomination because his positions - which include dramatic defense cuts and an isolationist foreign policy - are opposed by most party members.
Has romney `flatlined`?
When he finished second to Huckabee in Iowa in 2008, Romney won 25 percent of the vote in the state.
On Tuesday he received roughly the same percentage of the vote. Despite being the front-runner in the Republican race, Romney has not risen above 25 percent in national Republican polls.
Many Republican strategists say that is a problem.
"Mitt Romney has flatlined," said Ford O`Connell, a Republican strategist. "Obviously, he emerges as the odds-on favorite to win the nomination. But (Tuesday) was a good night for him, not a great night."
O`Connell said that for Republicans, "the key question is, Can Rick Santorum convince South Carolina and beyond that he has general election appeal? Will the anti-Mitt vote consolidate behind one candidate?"
Romney`s team is planning a trip to South Carolina this week, after Romney spends some time campaigning in New Hampshire.
Romney`s team also bought television advertising in Florida on Tuesday. Florida holds its primary on January 31.
In South Carolina, Romney has the support of the state`s Republican governor, Nikki Haley.
Chad Connelly, chairman of the South Carolina Republican Party, said Romney`s problems with social conservatives are not as significant as they were four years ago.
Romney will have to devote time and energy to doing well in what is called "the first in the South primary," Connelly said.
Connelly added that he was "pleasantly surprised" to learn that Romney would visit the state before the January 10 New Hampshire primary.
"That`s a pretty big deal for our state," Connelly said.
From Romney`s view, analysts said, the question will be whether conservative Republican primary voters remain divided between his rival candidates - which he would welcome - or one main challenger emerges.
Other analysts say Romney has to be prepared for sharp jabs by an inspired Gingrich - attacks that could eventually wind up aiding the Obama campaign`s efforts to define Romney as an opportunist who has changed his positions on a range of issues, including healthcare reform.
"What comes out of Iowa is not a clear picture," said Tad Devine, a Democratic strategist. "Romney is a guy who got 25 percent of the vote four years ago. There is a lot of incentive for the (other Republicans) to keep going."