Two GOP candidates need to drop out

Republicans are well positioned to win the 2016 presidential contest for a number of reasons: Our economy is soft, a majority of the public is deeply frustrated by the pace of growth and President Obama's job approval rating is stuck in the mid 40's. This is never good for the incumbent party.

What's more, the leading Democratic candidate, Hillary Clinton, is seen as dishonest and untrustworthy by a majority of the general electorate, especially Independents. On it's face, Republicans should win in November.

But, primaries matter. And, right now the GOP field is too crowded to ensure the party puts forward its strongest, most electable candidate. The bottom line is at least a couple candidates need to drop out after the South Carolina primary.

Spencer Platt | Getty Images

Republican presidential candidates (L-R) Ohio Governor John Kasich, Jeb Bush, Sen. Ted Cruz (R-TX), Donald Trump, Sen. Marco Rubio (R-FL) and Ben Carson stand on stage during a CBS News GOP Debate February 13, 2016 at the Peace Center in Greenville, South Carolina.

South Carolina has always played a pivotal role in the party's nominating contests and with the exception of 2012, it has always picked the GOP nominee. The polling in South Carolina has been all over the board. But, it looks like Donald Trump will win Saturday's contest with around 1/3 of the vote.

What happens below Trump will be much more interesting and determinative. There are two buckets of candidates: Ted Cruz and Ben Carson, and three mainstream Republicans, Marco Rubio, Jeb Bush and John Kasich.

Clinton? Rubio? Who's winning the Latino vote?

In the first bucket, Ben Carson needs to drop out. He's unlikely to break 10 percent in South Carolina and his campaign is over. Right now, his presence is harming the Republican Party's chances of nominating a strong general election candidate.

Ted Cruz will likely run a strong second or third in South Carolina and he'll remain a player in the upcoming primaries. He and Trump are in a micro contest for who's going to carry the "conservative" banner in later contests.

As the rules for upcoming primaries become stricter, meaning only registered Republicans vote, Cruz is more likely overtake Trump.

Time for this GOP candidate to drop out

Second, after the results are in Saturday, at least one of the three mainstream candidates will need to drop out or we will remain in a scenario where Donald Trump continues to win most contests with a weak plurality of the vote.

Bush or Kasich will need to be the bigger person, unless Marco Rubio finishes fourth. Both governors say they are going the distance and both have reasons to stay in as their delegate-rich winner-take-all home states are due to vote March 15th.

But, there isn't enough space for all three men. If they are still in the race on March 15th, hello nominee Trump.

Op-Ed: Trump should have run as a Democrat

The sooner the GOP race is two contests - one between Donald Trump and Ted Cruz and the other between two remaining mainstream candidates – the sooner Republicans will arrive at a place where they can take on Hillary Clinton and the Democrats with a winning candidate.

Commentary by Sara Taylor Fagen, a partner at DDC Advocacy and a former Political Director for President George W. Bush. She is also a CNBC contributor. Follow her on Twitter @sarafagen2.

For more insight from CNBC contributors, follow @CNBCopinion on Twitter.