Businessman Jon Delaney handily won the Democratic primary against one-time front-runner and state Sen. Rob Garagiola, setting up a battle with long-term Rep. Roscoe Bartlett (R) in a newly drawn Maryland district.

With two-thirds of precincts reporting, Delaney led Garagiola by 55 percent to 26 percent.

Garagiola was the favored candidate of most in the Maryland Democratic establishment, and had the backing of Gov. Martin O'Malley (D). But Delaney vastly outspent him and was boosted by endorsements from former President Clinton and The Washington Post.

Bartlett, a longtime member, is one of Democrats' top targets this fall: his newly drawn congressional district leans Democratic and is about half new to the centrist longtime congressman.

He was leading his primary with 45 percent of the vote against various candidates with about two-thirds of precincts in — a sign of how the unfamiliar territory might hurt him in the fall.

Bartlett is in his early 80s and was widely expected to retire, but has fought on and beat back a conservative challenge in his primary. It remains to be seen if he can do the same in an uphill battle for the general election.

The chairman of the Democratic Congressional Campaign Committee, Rep. Steve Israel (N.Y.), congratulated Delaney on his win and said Bartlett was "extremely vulnerable."

“John is a proven problem solver with a strong record of job creation and community service who will fight to protect Medicare and reignite the American dream for middle class Maryland families," he said in a statement. "We are confident that John will win in November."

Republicans, though, signaled they were confident Bartlett would survive the challenge. 

In a statement, Paul Lindsay, communications director of the National Republican Congressional Committee, said Bartlett had "proven he is not afraid of a tough fight."

“Meanwhile, liberal Democrat John Delaney is coming out of a nasty, bruising primary with state senator Rob Garagiola that has worn him out and exposed many of his flaws,” added Lindsay.

The Hill rates this race lean Democratic.

—This story was updated at 8:20 a.m.