With the Republican field narrowing, candidates upped their attacks on one another and on President Obama, courting groups ranging from evangelical Christians to the Tea Party from New Hampshire to South Carolina.

It is a critical time for candidates as they scurry to establish themselves as the best candidate to beat Obama, the trait most Iowa voters said they want to see in the Republican candidate. The six candidates will go on the national stage for the first time after the Iowa caucus at Saturday's debate, sponsored by ABC News, Yahoo and WMUR.

Unlike Iowa, where Ron Paul, Mitt Romney and Rick Santorum were tied in a three-way race in the last few days, Romney's win in New Hampshire appears to be clear cut.

He has more than double the support of his closest rival in the state that will host the nation's first primary. In a new  poll released today by the Union Leader, 47 percent of New Hampshire GOP primary voters said they would vote for Romney, compared to 17 percent for Paul and 13 percent for Huntsman.

But his frontunner status places Romney even more in the crosshairs. Following a scarring defeat in Iowa as a result of Romney's scathing negative campaign against him, Newt Gingrich is taking his gloves off and directly engaging with the former Massachusetts governor.

Gone is the pleasant Gingrich caucus goers saw in Iowa. The former House speaker is pulling out the guns, as evident in a new 30-second TV ad his campaign rolled out today in South Carolina, painting Romney as a "timid" candidate who "won't create jobs, and timid certainly won't defeat Barack Obama."

"I will defend free enterprise. I won't be confused about it, won't be timid about it, I'm prepared to be bold, this is Reagan's bold," Gingrich said at a press conference today in Plymouth, N.H. "I am going to continue making the case that there is a huge difference between a Reagan conservative and a Massachusetts moderate, and I think that resonates. That's the central argument for this campaign and will be the central argument in South Carolina and in Florida."

Jon Huntsman, who desperately needs a win in New Hampshire, where he has concentrated all his efforts, is also doing everything he can to strip away votes from Romney.

Then there's Rick Santorum. Romney may have won eight more votes than the former congressman in the Iowa caucuses, but Santorum is successfully tapping into the grassroots energy and enthusiasm that has turned him into a serious contender after months of dismissal.

"You may not agree with me on every issue and I suspect you don't, but I can tell you that I agree with me on every issues," Santorum said today, taking a not-so-veiled swipe at Romney's flip-flops.

Santorum will face his first real vetting this Saturday when he takes on the national stage for the first time as a contender who has a real chance at defeating Romney. He may lack the money advantage when compared to his rivals, but that could change quickly. Santorum raised $1 million in the last 24 hours alone, almost exclusively from small-dollar contributions, and the surge in Iowa gave him a much-needed boost to his national image that he will need to do well in other states.

But he will also have to deal with a new line of attacks, most notably from Sen. John McCain, who endorsed his former rival Romney on Wednesday.

McCain went at his former colleague hard today, attacking him specifically on defending earmarks.

"I believe that earmarking is a gateway drug to corruption. Senator Santorum supported it and engaged in it as much as he possibly could. I strongly disagreed with it," the former GOP presidential candidate said in a CNN interview.

Santorum also has to step up campaigning in Florida, which will hold its primary later this month. The state has already begun to mail out absentee ballots, signaling the start of the voting season. Santorum barely has any staff in the Sunshine State and that will need to change soon if he wants to gain ground there.

The real test for the GOP candidates will be in South Carolina, which experts say is more representative of the national Republican mindset and is a valuable indicator of what's to come.

Romney has stepped up his on-the-ground efforts in the Palmetto State, but so have Santorum, Perry and Gingrich, with Perry skipping New Hampshire altogether.

Ron Paul is stepping up his game as well. As ABC News' Jonathan Karl points out, other than Mitt Romney, no other candidate has more resources now to wage a drawn-out campaign. Paul's campaign raised a whopping $13 million in the fourth quarter, behind only Romney's fundraising of more than $20 million.

"Anybody can win this in the next 16 days," South Carolina Republican Party Chairman Chad Connelly told the West Ashley Patch. "These next days are going to be the tale. Whoever gets here, does the best job of getting to know voters and getting out to the diners and just really connecting is going to win our state."

ABC News' Amy Walter, Elicia Dover and Shushannah Walshe contributed to this report.