44. Barack Obama (D) - Illinois Senator
43. George W. Bush (R) - Texas Governor
42. Bill Clinton (D) - Arkansas Governor
41. George H.W. Bush (R) - Ronald Reagan VP
40. Ronald Reagan (R) - California Governor
39. Jimmy Carter (D) - Georgia Governor
38. Gerald Ford (R) - Richard Nixon VP
37. Richard Nixon (R) - Dwight D. Eisenhower VP
36. Lyndon Johnson (D) - John F. Kennedy VP
35. John F. Kennedy (D) - Massachusetts Senator
First off I want to say the reason why I only go back to the last ten Presidents is because American politics as we know was very different before Kennedy became President. If you look at the list, four out of the ten Presidents were Governors before they became President. Four out of ten were Vice Presidents before they became President, and only two were Senators.
So what does this tell us about 2016. Well since Republican's don't have an incumbent Vice President and there is no former Vice President out there that will run for President we can cross that off the list. Now we are left with Senators, Representatives, and Governors. The only two Presidents in history that were a representative before they became President were James Garfield and Abraham Lincoln. So the chances of a Representative becoming the Republican nominee or even the President in 2016 is very slim. As for Senators, they have a really good chance of getting the Republican nomination but a less of a chance to become President simply based off the trend. So chances are the Republican nominee in 2016 will probably be a Governor.
People tend to be more comfortable electing Governors to the White House because the job description for being a Governor and a President is very similar. They are both executives but Governor are just on a smaller scale.