Virginia Gov. Robert F. McDonnell (right) and Lt. Gov. Bill Bolling. (Steve Helber/Associated Press)

In separate prepared statements, issued via e-mail within the space of half an hour, McDonnell and Bolling described the oath as a well-intentioned measure that could wind up hurting the party’s efforts to expand its base.

Late last month, the state Board of Elections approved the oath, a form for primary voters that reads: “I, the undersigned, pledge that I intend to support the nominee of the Republican Party for president.”

“The effect of the oath could be one of diminishing participation in the primary, at a time when our Party must be expanding its base and membership as we head into the pivotal 2012 general elections this fall,” McDonnell’s statement said. He also called the oath “unenforceable.”

“I am concerned that requiring a Loyalty Oath may send the wrong message about our desire to grow our party and create an opportunity for more people to become involved in the party,” Bolling’s e-mail said.

Also Thursday, the American Civil Liberties Union of Virginia urged the state GOP to drop the pledge requirement and threatened to file a lawsuit if it did not do so.

While acknowledging that political parties have the right to establish their own rules for membership and participation, state ACLU Director Kent Willis said, “This is a primary organized, operated and funded by the government, and the government cannot require voters to pledge support for a particular candidate.”

Prospects for the oath dimmed further last week, when Pat Mullins, chairman of the Republican Party of Virginia, announced that the state central committee would revisit the oath at a special meeting Jan. 21.

The state party proposed but later withdrew a similar oath for the 2008 presidential race.

In their statements, McDonnell and Bolling both suggested that the oath was meant to help the GOP maintain control of its primary against potential Democratic mischief-makers, who have no nomination fight of their own and can vote in the Republican primary because voters do not register by party in Virginia.

“I realize that one of the challenges with Virginia’s current open primary system is the possibility that our primary could be influenced by Democrats or other voters who do not have the best interest of our party or candidates at heart,” Bolling wrote. “That is a legitimate concern and that is why I have always supported and continue to support voluntary party registration in Virginia.”

Said McDonnell: “I know the loyalty oath was proposed as a possible good faith solution to this issue in this primary election, but there are other ways. I would support legislation to establish voluntary party registration in Virginia. Such a reform to our electoral system would eliminate the need for any oaths or pledges and greatly simplify the nomination process in the Commonwealth.”

This post has been updated since it was first published.