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Mr. Sarvis, a lawyer and software developer who as recently as 2011 ran for a state Senate seat in Northern Virginia as a Republican, has made clear he has no plans to throw his support behind another candidate in advance of the election. If he draws 10 percent of the vote on Election Day, he will secure the Libertarian Party automatic ballot access for the next several years.

“Look, Virginia voters are apathetic because the system is dysfunctional,” Mr. Sarvis said at a recent WRIC/Radio One forum. “I’m the only candidate who respects both your economic freedom and your personal liberty.”

Libertarian sympathies can be complicated. About 89 percent of self-described libertarians say they have negative views of the Democratic Party, and about 40 percent have unfavorable opinions of Republicans. A majority — 57 percent — hold favorable views of the Republican Party, according to the annual American Values Survey released this week by the nonpartisan Public Religion Research Institute.

Libertarians are also principled but not always easy to pigeonhole: A majority of libertarians support legal marijuana but not gay marriage, and would allow doctor-assisted suicide but wouldn’t raise the minimum wage. And they really don’t like Obamacare.

Mr. Cuccinelli recently has been making a clear push for libertarian-minded voters who might be leaning toward Mr. Sarvis, billing himself as “the most liberty-minded statewide elected official in my lifetime.”

But a poll released Wednesday by Hampton University’s Center for Public Policy suggests that Mr. Sarvis might not be losing his base at all.

The Libertarian drew 12 percent in that poll — up from 8 percent last month. The poll, of 800 likely voters and a 2.9-point margin of error, also gave Mr. McAuliffe an advantage of 42 percent to 36 percent over Mr. Cuccinelli.

The Democrat maintains about an 8.3-point advantage in RealClearPolitics’ average of public polls ahead of what is expected to be a frenzied final weekend of the campaign.

Former President Bill Clinton, President Obama and Vice President Joseph R. Biden are making appearances on behalf of Mr. McAuliffe at get-out-the-vote rallies in the closing days of the race.

The events are mainly in Democratic strongholds, such as Northern Virginia and Richmond, as well as college-heavy areas such as Virginia Tech in Blacksburg and the University of Virginia in Charlottesville.

Mr. Cuccinelli, meanwhile, has maintained a relatively low profile in Northern Virginia, instead trying to fire up his own base in places like southwestern Virginia and the Richmond suburbs.

Former Sen. Rick Santorum of Pennsylvania, who is immensely popular among “values voters” and evangelical Christians, will swoop into the state this weekend with his own army of supporters to knock on doors and get out the vote for the Republican candidate.

Mr. Cuccinelli also will get a boost from Wisconsin Gov. Scott Walker on Saturday and Sen. Marco Rubio of Florida, who attended a Richmond fundraiser for the Republican last month, on Monday.