When is nearly 70 percent support from a key voting bloc not good enough? When you’re a Virginia Democrat running statewide without President Obama atop the ballot.

As he seeks to reclaim the governorship for his party, businessman Terry McAuliffe isn’t drawing the same level of African American backing other Democrats have recently enjoyed, a new Washington Post poll shows.

McAuliffe is supported by 69 percent of black voters, compared with 10 percent for Attorney General Ken Cuccinelli II (R). And 20 percent of black voters say they have no opinion or would vote for neither candidate. By contrast, Obama received 93 percent of black votes in Virginia last year, helping to compensate for the loss of the white vote to Mitt Romney (R) by 24 points.

Obama also won majorities of women, young voters and Northern Virginia residents, all of whom are showing less enthusiasm for McAuliffe, according to the poll. But no group supported Obama — and other Democrats — as overwhelmingly as African Americans.

State Sen. R. Creigh Deeds (D-Bath) received 90 percent of the black vote in his 2009 gubernatorial race, even as he lost to Robert F. McDonnell (R) by a wide margin.

The off-year electorate in Virginia is different from presidential election years, and the last nine Virginia gubernatorial races have been won by the party that did not hold the White House. As Democrats look to buck the trend, they must figure out how to convince their base to again show up at the polls in droves. That means erasing an apparent enthusiasm gap.

In a Washington Post poll in September, white and nonwhite voters were about equally likely to say they were certain to cast a ballot in the November election — 86 percent of nonwhites and 90 percent of whites. But now, 72 percent of whites are certain they’ll vote, compared with 56 percent of nonwhites.

Michael McDonald, an associate professor of government and politics at George Mason University and an expert on voting patterns, said that black turnout fell more sharply than did white turnout in elections after 2008, and that’s likely to happen again.

“We should not expect the 2012 electorate to be replicated in 2013,” McDonald said. “You can never say never, but the expectation should be that we would see a larger drop-off in turnout among African Americans.”

McAuliffe might be suffering from a familiarity problem. Only 29 percent of black voters said they knew a lot or a fair amount about McAuliffe and his qualifications to be governor, while 37 percent said the same of Cuccinelli.

Jared Armand Keys, a defense contractor who lives in Chesapeake, is an independent who voted for Obama last year. But he said he is “neutral” on McAuliffe and is unsure how he’ll vote this fall.

“I just want to find out a little more about their platforms,” said Keys, 46. “I’m just beginning to delve into it right now.”

Jordan Winston, 69, of Afton, said he votes regularly — even in off-year elections — and typically casts his ballot for Democrats but that he will vote Republican “if the Democrats ain’t got nobody running.” Like Keys, he backed Obama last year, but his gubernatorial vote is up for grabs.

“I haven’t made that decision yet,” Winston said. “I haven’t heard much about either one of them. I just ain’t been keeping up with it. I don’t know who’s running, what they’re saying.”

Even some African Americans backing McAuliffe aren’t necessarily enthusiastic about it.

“I’m choosing the best of the worst,” said Jeanneatha Nixon, 66, a restaurant manager from Suffolk. She doesn’t think much of politicians, generally viewing them as selfish — except for Obama.

“I see him differently,” Nixon said.

Jon Cohen, director of polling for Capital Insight, the independent polling group of Washington Post Media, and Capital Insight pollsters Scott Clement and Peyton M. Craighill contributed to this report.