CHARLESTON, S.C. - Memo to Republican presidential candidates not named Mitt Romney: This could be your last best chance.

With the number of candidates dwindling  after Rick Perry's departure from the race today, with Newt Gingrich surging in South Carolina polls, with Rick Santorum now the  winner of the Iowa caucuses  (albeit uncertified), Romney - already enduring a difficult week after disclosing his 15 percent tax rate - may never be more vulnerable than he is now.

For the three remaining candidates hoping to knock Romney off his perch at the top of the Republican primary, the clock will be ticking, not just on their comments from the podium  but possibly on their time left in the overall race.

Fresh off his resounding win in New Hampshire, Romney rolls into Saturday's South Carolina primary knowing that success here will virtually lock up the nomination for him. Another win in Florida's primary on Jan. 31 would be the nail in the coffin.

But the rapidly changing GOP race may now be swaying in the direction of Romney's opponents . While some of his rivals have been left in the dust in recent weeks - Michele Bachmann dropped out after Iowa, Jon Huntsman after New Hampshire and now Rick Perry before South Carolina - that means his remaining opponents are likely to go on the offensive in the hope  of seizing the opening. If they allow Romney to emerge largely unscathed at tonight's debate - the way they did in debates before the New Hampshire primary - then they may not get another chance to change voters' minds. But if they can get the former Massachusetts governor off his game  the way they did to some extent in Monday's debate in Myrtle Beach, they might be able to jump-start a race that a few days ago seemed an easy victory for Romney.

In the past few days, there have been signs that Romney's lead in the Palmetto State has shrunk. According to a new CNN/Time/ORC poll released before the debate, Romney leads Gingrich 33 percent to 23 percent among registered Republicans likely to vote. Santorum, after his impressive finish in Iowa, sits in third at 16 percent, followed by Paul at 13 percent and Perry at 6 percent. The poll was conducted Jan. 13-17, before Gingrich's strong performance in Monday's debate and Romney's revelation that he pays a tax rate of 15 percent.

The results indicate a tightening rate in South Carolina. In early January, Romney held a commanding 37 percent to 19 percent lead over Santorum, with Gingrich at 18 percent, but the former House speaker is now making a late surge, a welcome development for his campaign after its disappointing showing  in the first two voting states.

Gingrich has gone as far as to acknowledge that a victory for Romney would virtually lock up the GOP primary for Romney, even while saying that he intends to move on to Florida come Sunday no matter what happens the day before. Gingrich this week called for Santorum and Perry to drop out, saying that such moves would help consolidate the conservative opposition to Romney. Not only did the Texas governor today withdraw from the race, but he went so far as to endorse Gingrich, whom he described as "a conservative visionary who can transform our country."

In a statement responding to Perry's endorsement, Gingrich said that "South Carolinians have a chance this Saturday to nominate a bold Reagan conservative who will offer a dramatic contrast to President Obama this fall in the general election."

Santorum, however, has responded with defiance and indignation to Gingrich's request. After all, Santorum today was revealed as the winner of the Iowa caucuses - what was initially thought to be an eight-vote victory for Romney was actually a 34-vote win for the former Pennsylvania senator.

Despite Santorum's defiance, Perry's departure might be enough to provide a much-needed boost for Gingrich before Saturday's primary; the dwindling number of candidates, especially now that Perry is out, should only help Republican voters to coalesce around an alternative to Romney.

Even before today's developments, the Romney campaign had seen the margin over Gingrich narrowing - on Wednesday the campaign held a conference call that tried to link Gingrich to President Obama. Even with the suddenly close battle for South Carolina, the Romney camp will likely take heart in the latest poll results out of Florida. The CNN/Time/ORC poll revealed the former Massachusetts governor with a commanding lead there, up 43 percent to 19 percent over Santorum, with Gingrich in third at 18 percent.

The Florida primary is set for Jan. 31. Wins in South Carolina and Florida would all but secure the GOP nomination for Romney, but first things first: The battle for South Carolina is far from over.

Matthew Jaffe covers the 2012 campaign for ABC News and Univision.