Virginia Attorney General Mark Herring (D), left, and Republican attorney general nominee John Adams. (Reuters and Bill O'Leary/The Washington Post/Reuters and Bill O'Leary/The Washington Post)

The race for Virginia attorney general got a lot more expensive Thursday as the biggest donors to both campaigns announced more than $500,000 each in new contributions to their party's candidates.

The Republican Attorneys General Association donated $550,000 to John Adams (R), who is trying to unseat first-term incumbent Mark R. Herring (D) on Nov. 7. The Republican group has now contributed about $4.1 million to Adams, a conservative Richmond lawyer.

The Democratic Attorneys General Association was planning to announce later Thursday that it has pumped an additional $725,000 into Herring's campaign fund — a $500,000 donation made last week and $225,000 this week — bringing its total cash contribution to $2.25 million. The group has also made in-kind contributions worth about $470,000 to the progressive incumbent.

With 12 days to go until the election, Herring has the financial advantage in the race, with $6.2 million raised to Adams's $3.4 million, according to the nonpartisan Virginia Political Access Project.

Most polls have shown Herring with a narrow but consistent lead in what has traditionally been a low-turnout election.

The Republican attorneys general group signaled earlier this year that it would break a long-standing practice of not spending money on races in which one candidate is an incumbent. Its Democratic counterpart quickly followed suit. The Virginia contest is the only attorney general's race in the nation this fall.

Correction: An earlier version of this article had incorrect figures of $750,000 in new Democratic donations and $2.5 million in total cash contributions. The numbers have been revised.

Virginia general election guide