Sen. Marco Rubio's, R-Fla., presidential campaign has been facing a problem.

Though, on paper, Rubio is seen as the candidate most ideally positioned to unify the so-called establishment and conservative wings of the Republican Party, in practice, he's been hitting a ceiling in polls.

Rubio finds himself within the top three positions both nationally and in early primary states, but he's well behind the top spot, with his numbers stuck in the low double-digits. There's no one state among the early nomination contests where he has an obvious path to victory.

It's true that, just about a month out from when voting starts with the Feb. 1 Iowa caucuses, there's still time for Rubio to make a move. In 2012, for instance, the eventual Iowa winner, Rick Santorum, didn't even break out of the single digits in polling until about a week before the caucuses.

But so far, Rubio has had a difficult time breaking through Donald Trump's media dominance, or finding an effective attack against Sen. Ted Cruz, R-Texas. Efforts to portray Cruz as an isolationist or as disingenuous on immigration policy have not done anything to hinder the surging Cruz, now the Iowa frontrunner.

One of the core themes of Rubio's campaign — "yesterday is over" — may have been a strong basis for attack when Jeb Bush was seen as his chief rival. But it has much less resonance against Cruz, who is just six months older than Rubio, or Trump, who while from an older generation, certainly doesn't fit the mold of a traditional Republican candidate of yesterday.

That's where Hillary Clinton could come in.

One of the strongest cases for a Rubio nomination is electability. He consistently performs better against Clinton than all of his GOP rivals in hypothetical general election matchups.

What's more, there's evidence that he'd do better among key demographic groups. One recent NBC/WSJ poll even found Rubio tied with Clinton among young voters.

To put the significance of this in perspective, keep in mind that in 2012, when Obama was reelected with 51 percent of the national vote, he carried 18-29 year olds by a staggering 23-point margin.

If Rubio came anywhere near Clinton with younger voters, it would be game over for Democrats. This prospect prompted liberal Washington Post writer Greg Sargent to suggest recently that Clinton should be worried about Rubio.

Up until this point, however, Rubio's problem is that the argument about electability hasn't played much of a role in the GOP primary fight.

Part of that may be that the prospect of a second Clinton presidency still seems abstract and that she's still viewed as a weak candidate who is facing a tougher than expected challenge from a septuagenarian socialist in the Democratic primary.

The entire dynamic of the GOP race could be changed if Clinton delivers an early knockout blow against Sen. Bernie Sanders, I-Vt. Clinton is currently favored to win comfortably in Iowa, but trails Sanders in New Hampshire. The New Hampshire polls could change overnight with a big Clinton win in Iowa. And with a Clinton win in New Hampshire, the Democratic race will effectively end.

It's one thing for Republicans to know that Clinton is highly likely to be the Democratic nominee. It's another thing to watch a triumphant Hillary Clinton at a victory rally, side by side with Bill, looking like a dominant political force ready to bulldoze her way into the White House.

Facing the very real prospect of a Hillary Clinton presidency could re-focus the priorities of Republican primary voters. Those who haven't thought much about electability may suddenly consider it more important. Those who already prioritize electability may decide to consolidate support among one candidate with increased urgency.

To be sure, voters who place a premium on electability could still decide to vote for Trump, because they think he'd be more aggressive against Clinton; or they could go with Cruz, because they believe that winning the presidency is about motivating the party's base; or they could be attracted to one of many other candidates.

But, of all the arguments currently available to Rubio, his competitiveness against Clinton is the strongest. So the more worried Republican voters are about Clinton, the more it plays into his strength.