Many expected that the billionaire Paul Singer's public endorsement of Marco Rubio earlier this year would lead to the GOP establishment to mobilize behind the Republican candidate but that hasn't happened, NBC News reported.
The party remains fractured, with many supporting New Jersey Gov. Chris Christie while others remain undecided, like most U.S. House Republicans and former Massachusetts Gov. Mitt Romney.
NBC News looks at some of the reasons -- like inexperience -- why Rubio hasn't gained the backing of more players.