Democrats have captured governors' seats from Republicans in New York, Ohio and Massachusetts, broadening their hold on big-state power bases ahead of the 2008 presidential election.
New York elected Democratic Attorney General Eliot Spitzer to replace departing Republican Governor George Pataki, according to media projections.
Mr Spitzer's victory restores the governor's seat to Democratic hands for the first time in a decade.
In Ohio, decisive in the 2004 White House race, Ted Strickland, six-term congressman and Methodist minister, was projected as the first Democrat in 16 years to be elected governor.
Massachusetts, one of the most faithfully Democratic states in presidential elections, returned the governor's job to the party also for the first time in 16 years by electing Deval Patrick as its first black governor and only the second black governor in any state.
He was elected to succeed incumbent Mitt Romney, who did not run for a second term and is expected to make a presidential run in 2008.
In other projected returns from the 36 states electing governors, Democrats were re-elected in Illinois, Michigan, Pennsylvania, New Hampshire, Oklahoma, Wyoming, New Mexico and Tennessee, while Republicans incumbents were returned to office in Georgia, Nebraska, Connecticut and Vermont.
The stakes are high because control of the highest elected office in each state sets the agenda for policy making at state level and provides political leverage, money and influence to the candidate or their party in presidential election years.
Some analysts have suggested a popular governor can be worth 2 percentage points or more in their state in a presidential election.
Republicans, who have held a majority of the 50 governorships since their party's 1994 congressional landslide, went into the election holding 28 states, compared to 22 for the Democrats.
The pre-election speculation on governors' races was that Democrats could make a net gain of four states, putting them in a majority. The Republicans went in with a disadvantage theoretically, having to defend more seats than the Democrats, and facing what polls said was a pro-Democratic sentiment among voters generally.
Of the 36 races, Republicans were defending 22 seats, including nine where the Republican incumbent was not running. Democrats hold the other 14 seats but all except one were races with incumbents seeking re-election.
The races include nine of the 10 most populous states. California, with the largest population, remained in Republican hands with Governor Arnold Schwarzenegger retaining his seat.
Ohio, Iowa, Colorado, Minnesota and Wisconsin, along with Florida were considered presidential swing states and among the biggest prizes in Tuesday's voting. Florida, whose outgoing governor, Jeb Bush, brother of President Bush, was precluded from standing for a third term, went to the Republican's Charlie Crist.