Democratic challenger John Delaney (D) upset 10-term U.S. Rep. Roscoe G. Bartlett (R) in the newly drawn 6th Congressional District.
Just after midnight Tuesday, with about 57 percent of votes counted, Delaney led 58 percent to Bartlett's 39 percent.
Various news organizations called the race for Delaney, including CNN and The Associated Press.
The battle between Bartlett, 86, a scientist and inventor who lives on a farm near Buckeystown, and Delaney, 49, a founder and chairman of commercial lender CapitalSource who lives in Potomac, was the most competitive congressional race in Maryland.
Bartlett issued a statement before 10:30 p.m. saying: “I would like to congratulate John Delaney on his victory and thank him for a hard-fought campaign."
“I just hope God makes me worthy of the opportunity you've given me,” Delaney told hundreds of supporters gathered at his victory party in Potomac around 10:30 p.m. Tuesday.
Delaney, who noted that he grew up in working-class family, said the election was not about him but being “committed to working together for the common good of the country for everyone that believes issues matter,” standing up for women's rights, veterans and improving the nation's economy.
Bartlett, in a concession speech to Frederick County Republicans at Jekyll and Hyde's Tavern in Frederick, said: “I had a choice a year ago. The easy choice was to retire.”
But Bartlett said he believed running again gave Republicans the best chance to hold the 6th District seat.
“It's been a huge honor to serve the 6th District. I'm 86 years old. Whatever happens tonight, I'm a winner,” Bartlett said.
Bartlett said he's ready to help with the transition in any way he can.
U.S. Rep. Steny Hoyer, the House minority whip, told the crowd at Delaney's victory party that Delaney was in the race for the right reasons and that he has never met a more capable candidate.
Democrats, who hold the governorship and majorities in the Maryland legislature, redrew the 6th District to make it easier for a Democrat to win.
The traditionally Western Maryland district now includes more Democrats, 192,820, than Republicans, 145,620, and the second highest number of unaffiliated voters among the state's eight Congressional districts at 94,074, behind the neighboring 8th District 8, which has 98,556.
The 6th District now includes only a third of Bartlett's home county of Frederick and none of Carroll County or counties to the east, but added roughly a third of Montgomery County.
Outside a polling place at Our Lady of Mercy Roman Catholic Church on Kentsdale Road in Potomac Art Greenberg said he voted for Delaney because “he is a successful businessman and at the same time seems to balance that with compassion and empathy for the less fortunate.”
But Greenberg, who said he is registered as a Democrat but is really an independent, said the way the Democrats gerrymandered the district “was really pathetic ... It's a bastardization of the Democratic process."
Delaney pulled off a surprise victory in the Democratic primary in April when he defeated state Sen. Robert Garagiola (D-Dist. 15) of Germantown, who was backed by much of the state's Democratic establishment.
Bartlett's trek across the 6th District Tuesday had taken him to Urbana by afternoon, where he stopped on his way to Montgomery County.
In Urbana a woman wished Bartlett luck.
“It's a challenge this time, but we'll see what happens,” Bartlett told her.
Bartlett started his morning in snowy Garrett County where he said election officials told him they were having the biggest morning turnout they had ever seen.
Bartlett said he believed the weather and interest in ballot questions were drawing lots of voters.
Voters who cast their ballots on Election Day often faced shorter lines than those who did during a five-day early voting period that was curtailed, then extended, last week to avoid coinciding with Hurricane Sandy.
More than 430,000 voted early in Maryland, almost twice the number who did in 2010.
Staff Writers Ryan Marshall and St. John Barned-Smith contributed to this story.