It's Sen. Marco Rubio (R-Fla.) vs. Sen. Rand Paul (R-Ky.).

The two most likely 2016 presidential candidates in the Senate will go back-to-back tonight in consecutive rebuttals to President Obama's State of the Union address. First will be Rubio offering the official Republican response; then will come Paul delivering the tea party's take.

And the difference in tone between their responses could be a good metaphor for the fight for the heart and soul of the GOP -- not to mention the party's 2016 nomination -- going forward.

"It surely is juxtaposition," said GOP strategist David Norcross. "I'd rather it not happen, but probably [it] is unavoidable, so the sooner the better."

Rand Paul will respond to Obama’s State of the Union. (AP Photo)

The GOP establishment has acknowledged since the party's losses in the 2012 elections that it has problems -- big problems -- that need to be dealt with. In response, it has pushed for the party to find some middle ground on issues like immigration -- a project that Rubio, as it happens, is spearheading.

On the other side are the likes of Paul, who continues to pull the party to the right and insists that compromising on core conservative principles is a non-starter. That resistance to the party's more pragmatic tone has manifested itself in the number of House Republicans who voted against John Boehner for House speaker, the House GOP's hesitation to pass a Hurricane Sandy relief bill without corresponding spending cuts, and the more recent backlash against a new GOP group aimed at nominating more electable GOP Senate candidates in 2014.

As it stands right now, the party is very much torn between those paths.

Make no mistake: These two men were chosen to speak tonight because they are the respective futures of the groups they are representing -- Rubio of the broader GOP and Paul of the tea party. And it just so happens that they are also their factions' frontrunners for president in 2016.

None of this is to say that Paul's speech will be as big a deal as Rubio's. The tea party response in 2012 (delivered by Herman Cain) wasn't widely covered or aired after last year's State of the Union, and networks have yet to say they will air Paul's response after Rubio's.

But there is certainly interest in Paul -- moreso than there was in Cain after his presidential campaign imploded or in Michele Bachmann in 2011 when she delivered the tea party's first SOTU response. And Paul's response does afford him a chance to grow his already-rising profile in the tea party community and continue to lay markers in advance of what could very well be a presidential run in 2016.

If Paul does run in three years, he's got the inside track on becoming the tea party's standard-bearer.

Rubio, meanwhile, is being billed as the GOP's next big thing and has earned plaudits from the party establishment for his early work on immigration reform.

Even as Rubio has taken a leading role in immigration reform, Paul has been stalking him on the right, threatening to out-conservative him on the issues of the day. And Rubio needs to mind his right flank in a party that has routinely demanded ideological purity. (Rubio joined Paul, as it happens, among the five Republicans who voted against the 'fiscal cliff' deal on New Year's Day.)

Paul emphasized Sunday that he's not trying to be the alternative to Rubio.

"I won't say anything on there that necessarily is like Marco Rubio is wrong," Paul said on CNN. "You know, I don't always agree, but the thing is this isn't about he and I. This is about the tea party, which is a grassroots movement, a real movement with millions of Americans who are still concerned about some of the deal making that goes on in Washington."

Added Tea Party Express founder Sal Russo, whose group organizes the tea party response: "There is great diversity in the tea party on social and cultural issues. ... I am delighted to see Rubio address immigration and Paul push his liberty agenda. It shows the tea party has many sides."

Paul may well not be looking to be the anti-Rubio, but his appearance tonight after Rubio comes at a time of significant dischord in the Republican Party. And both his path to relevance and his political inclination is more in line with the tea party's values than the broader GOP.

Paul could make a splash tonight by distancing himself from Rubio and the GOP establishment or he could play it safe and merely emphasize fiscally conservative values.

The former would be troublesome for the GOP but potentially helpful for Paul. That latter would be a good sign for the GOP but not so great for Paul's efforts to build his national profile.