They also turned back all four of Republican Governor Arnold Schwarzenegger's efforts to reshape state government.
Democratic Senator Jon Corzine easily won the New Jersey governor's seat after an expensive, mudslinging campaign, trouncing Republican Doug Forrester by 10 percentage points. Polls in the last week had forecast a much closer race.
Democratic Lieutenant Governor Tim Kaine won a solid victory in Republican-leaning Virginia, beating Republican Jerry Kilgore by more than 5 percentage points. Democrats crowed that Bush's election-eve rally for the former state attorney general only spurred more Kaine supporters to the polls.
In California, Schwarzenegger failed in his push to rein in the Democrat-controlled Assembly. All four of his ballot measures flopped: Capping spending, removing legislators' redistricting powers, making teachers work five years instead of two to pass probation, and restricting political spending by public employee unions.
Elsewhere, Texas voters overwhelmingly approved a constitutional ban on gay marriage, Maine voted to preserve the state's new gay-rights law, and Republican Mayor Michael Bloomberg easily clinched a second term in heavily Democratic New York.
Democrats said the results were the first steps toward bigger victories next year - when control of Congress and 36 governors seats are at stake - and for the 2008 presidential race.
"I believe national Republican politics ... really had an effect in Virginia and California," said Democratic Party Chairman Howard Dean. Voters "don't like the abuse of power, they don't like the culture of corruption. They want the nation to go in a different way".
"I believe national Republican politics ... really had an effect in Virginia and California"
Howard Dean, Democratic Party chairman
"It's not some type of trend," said Republican Governor Mike Huckabee of Arkansas, noting that both seats were won by Democrats in 2001 when Bush's popularity was high. Still, he acknowledged the defeats - and said they could help rally the Republican base next year. "I don't think anybody will be complacent now."
Both governors' races were marked by record-breaking spending and vicious personal attacks.
In Virginia, Kilgore's campaign ran an ad claiming Kaine, a death penalty opponent, would have refused to execute Adolf Hitler, while Forrester quoted Corzine's ex-wife as saying he had let down his family and he would let down New Jersey.
In California, where Schwarzenegger faces re-election next year, the special election was seen as a referendum on his leadership. His prospects for a second term darkened as all four of his ballot measures failed.