WASHINGTON -- House Democrats increased their efforts to defeat Rep. Rodney Frelinghuysen this year, adding challenger Mikie Sherrill to their list of top challengers.

Sherrill was one of seven candidates added Wednesday to the Democratic Congressional Campaign Committee's "Red to Blue" list. These 18 Republican-held districts are where the DCCC believes it has the best chance of winning in 2018 and represent more than two-thirds of the 24 seats the Democrats need to pick up in order to regain their House majority.

Such top-tier candidates get extra fundraising and organizational help for their campaigns.

"A Navy veteran and former federal prosecutor, Mikie Sherrill has spent her life serving our country," said Rep. Ben Ray Lujan of New Mexico, the DCCC chairman. "Mikie gets the job done and it's clear why she's been able to build one of the strongest, grassroots-driven campaigns in the nation."

Several other Democrats also are seeking the nomination to take on Frelinghuysen, the most recent being lawyer Michell Cobert of Morristown, a former New York state assistant attorney general.

"I believe the people in our district need a proven advocate who will put their actual needs ahead of partisan rhetoric," Cobert said on his Facebook page.

Others include Montclair State University lecturer Tamara Harris, former U.S. Army National Guard member Jack Gebbia and County College of Morris history professor Mark Washburne.

Frelinghuysen, R-11th Dist., the most powerful member of the New Jersey congressional delegation, voted for the House Republican health care bill that the Congressional Budget Office said would leave 23 million more Americans uninsured.

While he voted against the GOP tax bill that gutted the federal deduction for state and local taxes -- a tax break disproportionately used by New Jersey residents -- he supported the budget resolution that allowed Republicans to prevent a Senate filibuster and exclude congressional Democrats from deliberations and negotiations.

In November. the Cook Political Report, a Washington-based publication that tracks congressional races, downgraded Frelinghuysen's chances of being re-elected , saying the race now was a toss-up.

Democratic Gov.-elect Phil Murphy carried the district in last November's election and President Donald Trump won with less than 50 percent of the vote in 2016.

Frelinghuysen stepped up his fundraising, bringing in $1.2 million through Sept. 30, compared with $588,320 in the same period two years ago.

Sherrill raised $744,014 and Harris 459,607. The other Democrats did not report raising any money through Sept. 30, Federal Election Commission filings show.

Jonathan D. Salant may be reached at jsalant@njadvancemedia.com. Follow him on Twitter @JDSalant or on Facebook. Find NJ.com Politics on Facebook.