Republican presidential candidate Senator Marco Rubio. (Gary Cameron/Reuters)

The critical early-voting state of South Carolina is emerging as a crucial front in the rivalry between Sen. Marco Rubio (R-Fla.) and former Florida governor Jeb Bush. Expectations for Rubio are soaring, Republicans say, putting pressure on Bush in a state his brother and father won — a state which has earned a reputation as a firewall protecting front-runners from insurgent underdogs.

Rubio, an underdog for the nomination who faces opponents with longer political resumes and deeper roots in the GOP, is in some ways an unlikely bet for South Carolina. As of now, he is averaging under 5 percent support there in recent polling.

But for years, he has been laying a foundation, often informally, in the vital early nominating state where he will officially campaign for president for the first time on Saturday.

Rubio’s team is being led by a small army of seasoned South Carolina operatives. Bush is relying in South Carolina on two strategists who have spent most of their professional careers outside the state.

Bush, viewed as the early establishment favorite as he raises heaps of cash and moves closer to officially announcing his campaign, faces further challenges. Republicans say Rubio is running on a platform that caters well to the state’s political leanings and has carefully cultivated relationships there. Meanwhile, some Republicans have doubts about Bush’s strategy in the state.

Sen. Marco Rubio, who's running for president in 2016, is known for his stances on immigration and tax reform. Here's the Florida Republican's take on Obamacare, the Islamic State and more, in his own words. (Julie Percha/The Washington Post)

Adding to the stakes in South Carolina is Florida’s nominating contest, slated for just a few weeks later. Both candidates are eager to return to their home state with a win already under their belts.

Rubio’s big test on this, his third trip to South Carolina this year, will be a speech at a conservative summit in Greenville, where about a dozen other White House aspirants will also speak. Among them: Wisconsin Gov. Scott Walker (R) and Sen. Ted Cruz (Texas), who like Bush, is doing better than Rubio in early South Carolina polling.

Bush will not be there, but he has made his presence felt in South Carolina. He was there last weekend for the state GOP convention and has made two other stops in recent months, including a two-day, four-city swing in March.

A pair of longtime South Carolina operatives and former business partners — Terry Sullivan and J. Warren Tompkins — are respectively running Rubio’s campaign and supportive super PAC.

Tompkins’s decision to back Rubio is especially notable since he worked for the presidential campaigns of both George W. Bush and George H.W. Bush.

Sullivan and Tompkins are barred under the law from coordinating strategy with one another. But their familiarity with each other’s style makes for a formidable tag team effort, Republicans say.

Heath Thompson, who helped steer George W. Bush to victory in the 2000 South Carolina primary, is also on Rubio’s team. In addition, Rubio has also enlisted the work of a South Carolina-based digital consulting firm and recently hired a paid staffer in the state.

Bush has tapped Brett Doster, a Tallahassee-based GOP consultant who has known him for more than two decades, to lead his South Carolina team. Despite studying at the Citadel for four years, he lacks any on-the-ground political experience in South Carolina.

Bush has also hired Jim Dyke, a Charleston-based ad maker who worked on his brother’s campaign.

Barry Wynn, a former South Carolina GOP chairman who is supporting Bush, said that Doster and Dyke will be the start of a South Carolina team that is expected to grow in the coming weeks with more paid staff and volunteers. Despite not having as much experience in South Carolina, Wynn said that Doster’s long ties to Bush should help him.

“He’s really ideal,” said Wynn. “Normally you either get somebody that’s not from the state who knows the candidate very, very well or you get somebody from the state that doesn’t now the candidate really well.”

But some Republicans question Bush’s early maneuvering.

“Jeb is behind the other campaigns. In terms of actual on-the-ground machinations, they haven’t done a whole lot of anything,” said one senior South Carolina Republican who is neutral. The Republican was granted anonymity to speak candidly about the field.

Al Phillips, a Greenville-based pastor who met with Rubio and Bush in recent months and was impressed by both, said he noticed a difference in style.

“Jeb Bush seems a little more private, but that’s just his nature,” said Phillips. Rubio, he said, was “very outgoing.”

Beyond staffing, Rubio’s political platform presents Bush with a challenge. His attempt to appeal to both ends of the GOP spectrum — he was elected to the Senate as a conservative rebel but has since also endeared himself to the establishment wing — and his aggressive national security posture is ideal for South Carolina, Republicans say.

“He should do very well in South Carolina because they like that hybrid tea party-establishment person who is also a defense hawk,” said Ford O’Connell, who worked on the 2008 presidential campaign of Sen. John McCain (R-Ariz.).

Which raises another potential complicating factor for both Rubio and Bush: Sen. Lindsey O. Graham (R), a defense-focused home state candidate who may decide to run for president.

McCain, who won South Carolina in 2008, pointed to the state’s sizeable veteran population and the presence of several military bases as a reason why it’s crucial to emphasize hawkish views.

Asked where he stood in his decision-making process this week, Graham, said he was “98.6” percent sure he would run, teasing, “Stay tuned.”

Richard Quinn, a longtime Graham strategist, said many people misunderstand the South Carolina primary, believing it to be dominated by hard-right conservatives.

“What a lot of people don’t understand about South Carolina is our presidential primary is almost like a general election. We don't register by party,” said Quinn.

South Carolina has long been viewed as a state where leading candidates can separate themselves from the pack. Since the inception of the “First in the South” primary in 1980, the winner of South Carolina has gone on to claim the GOP nomination in every election except 2012, when Newt Gingrich defeated Mitt Romney there.

That 2012 result gives some hope to Rubio for competing there even if he does not emerge from the other early states as the leader of the pack.

In an election cycle when wealthy donors are expected to keep their preferred candidates afloat through super PACs, which can raise unlimited funds from individuals and corporations, candidates may be able to play a much longer game than ever before.

“Iowa tends to be more a caucus driven by the impact of social conservatives,” said Tompkins. “New Hampshire seems to be a primary driven by fiscal conservatives and independents and libertarians. In South Carolina, you have a strong combination.”

South Carolina Republican Party Chairman Matt Moore said Rubio has carefully tended to personal relationships in the state since his 2010 campaign for U.S. Senate. The first sitting Republican senator to endorse Rubio in that contested GOP primary was Jim DeMint (R-S.C.), now the president of a conservative think tank.

“He’s here not only on the record doing political events and fundraisers,” said Moore of Rubio, later adding, “he has done lunches, he talks to insiders here — and without staff interference.”

In July of 2014, Rubio was driving back to Washington from Florida when he decided to make a pit stop in South Carolina.

“He wanted to do something fun and spiritual,” recalled Sen. Tim Scott (R-S.C.). “So I invited him to church and [said], ‘Let’s do it.’ So we went to church and enjoyed the service.”

Still, Rubio’s spadework has not yielded anything concrete yet. He sits in ninth place in South Carolina, according to the Real Clear Politics average of recent polling. Bush is leading, and even long-shot Ben Carson has registered more support than Rubio.

And Bush appears to be raising money at a record clip, potentially giving him the means to flood the television airwaves in South Carolina. Television is an important mass communications medium in the state, which is more populous than Nevada, Iowa or New Hampshire.

But South Carolina Republican say a candidate’s ground game is just as important as the air war. And Rubio is regarded as a talented retail politician.

On Saturday, the senator plans to meet with business and religious leaders.

“The people have gotten spoiled by candidates showing up and actually shaking hands,” said Phillips.

For Rubio, Bush and all the Republican hopefuls, this means continuing to woo the many power brokers like Scott, who is neutral right now.

Ed O’Keefe and Philip Rucker contributed to this report.