Marco Rubio
Marco RubioReuters

After coming in second in the South Carolina primary, Florida Senator Marco Rubio won a flurry of endorsements, making him the undisputed winner of what political insiders call “the endorsement primary”.

Support from prominent Republicans helps candidates build momentum and sway the mass of undecided primary voters in their favor. Since the beginning of the modern primary system in 1976, every winner of the endorsement primary has gone on to secure the nomination.

Senator Rubio now enjoys the backing of 13 sitting US senators, 3 governors, and 42 US House representatives. His closest competition in the establishment primary, Texas Senator Ted Cruz, is only backed by 22 House representatives – with no senators or governors supporting his campaign.

Rubio’s benefitted greatly from the collapse of two rival campaigns – New Jersey Governor Chris Christie’s, and former Florida Governor Jeb Bush’s. Both had attracted significant support from prominent party members, most of whom have now shifted to Rubio.

Since Bush and Christie dropped out following poor showings in early primary states, Rubio more than doubled the number of establishment figures backing his campaign. Former Kansas Senator and one-time GOP presidential nominee Bob Dole, who had previously backed Bush, threw his support behind Rubio. Even 2012 nominee and former Massachusetts Governor Mitt Romney is rumored to be preparing a formal endorsement of Rubio.  

Cruz, on the other hand, has had little traction amongst a party establishment he’s worked so tirelessly to assail. Indeed, his campaign’s central theme has been a rejection of the GOP leadership, a stance that not surprisingly has earned him no backing from fellow GOP senators.

While Cruz is betting on this year’s wave of anti-establishment angst to propel him past Rubio to the nomination, Saturday’s primary vote in South Carolina does not bode well for him. A southern state with an overwhelmingly Evangelical GOP electorate is about as friendly ground as the senator will find in his bid for the nomination, yet he was narrowly bested by Rubio, thanks in large part to the last minute endorsement of South Carolina governor Nikki Haley.

Compared to previous GOP nominees, Rubio is in the middle of the pack. Nate Silver’s FiveThirtyEight tracked the weighted endorsements of past nominees, and found Rubio to have surpassed both Ronald Reagan at this point in his 1980 campaign and John McCain in 2008.