Marco Rubio finisehd second to Donald Trump in the South Carolina primary, barely beating Ted Cruz by two tenths of a percentage point.

In a celebratory speech to supporters, Cruz Rubio returned to the theme that he represents a "new generation of conservatives":

Ronald Reagan made us believe that it was Morning in America again. And it was. Well now the children of the Reagan revolution are ready to assume the mantle of leadership.
 [Applause] 

Now those of us who grew up when it was morning in America and Ronald Reagan was in the White House are ready to do for our generation - are ready to do for the next generation what Ronald Reagan did for ours. 
[Applause]

And who is this conservative movement in the 21st century? Because our principles remain the same. But who are we? My friends, the 21st century conservatives are the son of a single mother who grew up in poverty and was almost lost. And today he serves this state as its junior senator, Tim Scott. 
[Applause] 

The 21st century conservative movement is the daughter of immigrants from India who wanted desperately for their children to have all the opportunities they never did. Who faced a string of prejudice and yet because of the greatness of our country today, Nikki Haley is the governor of a state where it's always a great day. 
[Applause] 

And the 21st century conservative movement is the son of a bartender and a maid from Cuba who tonight stands one step closer to being the 45th president of the United States of America. 
[Applause] 

This is a powerful message to a certain segment of Republicans who want someone they believe can unite the two wings of the party. 

But Rubio has a huge problem. He exceeded expectations in Iowa and South Carolina, stumbling badly in New Hampshire. But after last night, the expectations game becomes nearly meaningless. The candidate must now break through somewhere - and somewhere soon - if he is to remain a viable candidate.

Until Rubio proves he can beat both Donald Trump and Ted Cruz, every second and third place finish from here on out will only serve to narrow his path to the nomination. This is especially true after the withdrawal of Jeb Bush from the race. 

On the one hand, his support from establishment Republicans make him the ideal heir to Jeb Bush's supporters (and perhaps more importantly, his donors). And just like he did in Iowa, he has shown the ability to close strong in races.

Yet he also faces major challenges. Among them: His best strength is electability, an attribute prized by few Republican voters this year. With Trump drawing wide support across party constituencies and Cruz pulling heavily from social conservatives, it's unclear whether Rubio can put together a winning coalition in many upcoming contests.

That's an extremely narrow road to the nomination. Both Cruz and Rubio are hoping that support for Trump tops out somewhere in the mid to upper 30's. In a three man race, that will open the door for both candidates to portray themselves as the only alternative to Trump that can win in November. 

Tuesday's Nevada caucus will be a test for Rubio's new found strength. Finishing a distant second is no longer relevant. From now on, it's a contest for delegates. And winning more delegates than Trump or Cruz becomes the only goal that matters for Rubio going forward.