Overview
Following West Virginia Gov. Jim Justice's switch to the Republican Party on Aug. 3, 2017, Democrats were at an all time low, holding just 15 governorships to 34 for Republicans and 1 Independent.  Democrats picked up New Jersey in the Nov. 2017 off-year elections.  In Nov. 2018 they had ample opportunity to make further gains.  Of 36 seats at stake, Republicans held 26, and half of those were open (12 term-limited and 1 retirement).  Four Republican incumbents ascended to the governorship and were seeking election in their own right: Gov. Jeff Colyer (KS) was defeated in the primary, while Kay Ivey (AL), Kim Reynolds (IA) and Henry McMaster (SC) were on the Nov. 6 ballot. 

There were many interesting primaries.  On the Democratic side, all five incumbents seeking re-election won; Gov. David Ige (HI) fended off perhaps the strongest challenge, from U.S. Rep. Coleen Hanabusa.  Crowded fields vied for the Democratic nomination in a number of states.  For example, in Maine and Maryland seven candidates were on the ballot and in Wisconsin eight candidates.  In California 12 of the 27 candidates on the ballot in the top-two primary were Democrats, although only six were seen as credible.  Texas saw a runoff on May 22; Lupe Valdez defeated Andrew White.  On the Republican side, the closest contest was the Aug. 7 primary in Kansas, where Secretary of State Kris Kobach (endorsed by Trump) narrowly defeated Gov. Jeff Colyer; Colyer conceded on Aug. 14.  There have been three runoffs.  Georgia Secretary of State Brian Kemp (endorsed by Trump) defeated Lt. Gov. Casey Cagle in the July 24  runoff.  In South Carolina Gov. Henry McMaster defeated John Warren on June 26, and in Oklahoma, businessman Kevin Stitt defeated Oklahoma City Mayor Mick Cornett on Aug. 28.  Tennessee also saw a very competitive Republican primary, with four strong candidates competing in the Aug. 2 election.

Among the states where Democrats hoped to achieve pickups were Florida, Georgia, Illinois, Iowa, Michigan, Nevada, New Mexico, Ohio and Wisconsin; they won five of those races and lost four.  Republicans eyed pickups in Connecticut and Minnesota, but succeeded in neither of those races.  Democrats ran a diverse field of candidates including 11 women and one transgender candidate.  In Georgia former House Minority Leader Stacey Abrams sought to become the first African American woman governor, in Florida Tallahassee Mayor Andrew Gillum sought to become the first African American governor of the state, in Colorado Jared Polis sought to become the first openly gay man elected governor (Jim McGreevey in New Jersey came out as gay in 2004 after he was elected and Kate Brown, openly LGBT, was elected in Oregon in 2014), and in Vermont Christine Hallquist aimed to be the first transgender governor.  The Republican field was rather more homogenous, including four women nominees.  Libertarians had more than 20 candidates for governor on the ballot, and Greens about eight.  The one independent governor, Gov. Bill Walker (AK), was seeking re-election but then suspended his campaign on Oct. 19.  Other notable independent candidates including Greg Orman in Kansas, Terry Hayes in Maine and Oz Griebel in Connecticut and Stephanie Miner in New York found themselves squeezed out by the major party candidates.  The Illinois campaign was the most expensive of the governors' races.

Links:  DGA  [Organization]   |  RGA  [Organization]
NGA   |   Center on the American Governor
Larry J. Sabato's Crystal Ball
The Cook Political Report