The challenged ballot shows bubbles for both Ms. Simonds and Mr. Yancey filled in, with a slash through the Simonds vote. Mr. Yancey’s lawyers argued in court last week that the voter intended to cross out the Simonds vote. The state handbook reads, “If there are identical marks for two or more candidates, clarified by an additional mark or marks that appear to indicate support, the ballot shall be counted as a vote for the candidate with the additional, clarifying marks.”

As further evidence of intent, Mr. Yancey’s lawyers pointed out the voter selected all the named Republicans on the ballot.

But in a motion this week, Ms. Simonds’s lawyers noted the bubble for the Republican candidate for governor, Ed Gillespie, was both blacked in and also marked with an X. Could an extraneous mark be both a sign of opposition to Ms. Simonds and of support for Mr. Gillespie?

“Nothing in the ‘Ballot Examples’ authorized this Court to presume that the slash mark in question should be read as a mark of opposition to Ms. Simonds,” the Democrat’s lawyers argued in their motion. The ballot should be thrown out as an “overvote,” they said.

To a layman, a ballot may be “like a Rorschach Test,” open to interpretation, Ms. Simonds’s team argued. But that is not the way it is, they said. Election officials must follow specific examples in the guidebook.

A Republican elections lawyer unconnected with the case, Jason Torchinsky, predicted the three-judge panel was unlikely to be persuaded to throw out the ballot. “It seems the Democrats in Virginia are now asking a court to deny a decision to count a vote,” he said by email. “Quite a change in position from their usual argument that every vote be counted.”

Ms. Simonds’s legal team further argued that the judges should never have agreed to consider the questionable ballot on Dec. 20, when it was brought to them one day after the official recount. The time to question the ballot was during the recount proper on Dec. 19, presided over by lawyers and observers for both candidates.