The American Conservative Union (ACU) is endorsing Virginia Del. Barbara ComstockBarbara Jean ComstockVirginia New Members 2019 Defeated Republicans mocked by Trump fire back at president GOP lawmaker defends Mia Love from Trump attacks: 'I was disgusted when I heard it' MORE (R) for retiring Rep. Frank WolfFrank Rudolph WolfVulnerable Republican keeps focus as Democrats highlight Trump Bolton could be the first national security chief to prioritize religious freedom House votes to mandate sexual harassment training for members and staff MORE's (R-Va.) seat.

The ACU is just the latest major conservative group to back Comstock, who is looking more and more like a lock to be the GOP's nominee in the slightly Republican-leaning district.

Her biggest threat for the nomination, controversial Virginia state Sen. Dick Black (R-Va.), dropped out of the race last week, and no other Republicans have jumped in against her.

"ACU PAC is proud to support Virginia state Delegate Barbara Comstock's candidacy for the 10th District congressional seat currently held by retiring Congressman Frank Wolf," ACU Chairman Al Cardenas said in a statement. "Barbara is a principled conservative with a strong record  who has earned a 97 percent lifetime rating with ACU.  As a former senior aide to Rep. Wolf, as well as a skilled legislator, attorney, small businesswoman, and spokesperson, Barbara is the obvious and best choice for this seat to represent Northern Virginia."

The ACU endorsement makes them the latest in a string of big-name supporters for Comstock, a top Republican lawyer, strategist and lobbyist. She has the backing of Mitt Romney, who she worked for in 2012, as well as conservative radio host Mark Levin, Citizens United and Americans for Prosperity President Tim Phillips.

Democrats are hopeful they can contest the seat and believe Comstock's work as a lobbyist could hurt her. National Democrats think Fairfax County Board of Supervisors member John Foust (D) could give her a tough race, though attorney Richard Bolger (D) is also running for the seat.