Boyd Marcus, a veteran Republican political strategist, broke with his party Tuesday to endorse Democrat Terry McAuliffe over Republican Ken Cuccinelli II in the race for governor of Virginia.

Marcus, who served as chief of staff to then-governor James Gilmore III and advised George Allen’s bids for governor and U.S. Senate, will work as a paid adviser to the Democrat’s campaign.

Marcus is not the first Virginia Republican to endorse McAuliffe, but some of those have crossed party lines before.

That is not true for Marcus, although he has previous opposed Cuccinelli, the current attorney general. Marcus served as chief strategist for David Foster in his bid for the GOP nomination for attorney general against Cuccinelli, the current attorney general, in 2009. Marcus also has worked closely with Lt. Gov. Bill Bolling (R), who abandoned his bid for governor after Cuccinelli got into the race.

“I am enthusiastically supporting Terry McAuliffe for Governor because I believe he will work with both parties to advance an agenda that prioritizes economic growth. Virginia needs an experienced businessman who will put the practical needs of our people ahead of political ideology,” Marcus said in a statement. “I’ve never before supported any Democrat, but this election Terry is the clear choice for mainstream conservatives.”

More Post coverage of the race for Virginia governor.

McAuliffe, who has highlighted Cuccinelli’s conservative stances on social issues, touted the endorsement as evidence that the Republican candidate is out of step with his own party.

“Boyd is a great addition to our campaign team that is focused on bringing together Virginians of both parties,” McAuliffe said in a statement. “Leadership means working with both parties towards compromises that move Virginia’s economy forward.”

Cuccinelli’s campaign said the endorsement will not matter to voters.

“Virginians are concerned about each candidate’s vision for the future and how they plan to grow the economy and create jobs, not who their political consultants are,” Cuccinelli spokesman Richard T. Cullen said in an e-mail. “Ken Cuccinelli has a long and distinguished record of leadership and service in Virginia and has laid out substantive plans that will make the Commonwealth stronger. Terry McAuliffe, meanwhile, has been exposed as a failed job creator who has misled Virginians at every turn about his business record.”