Flagstaff Democrat Ann Kirkpatrick pulled off a narrow victory over Marana Republican Jonathan Paton in the vast district that encompasses both their hometowns.
Preliminary results, with all precincts counted, gave Kirkpatrick 49 percent of the vote in Congressional District 1, to Paton's 45 percent.
The 7,227-vote margin is likely too large for Paton to overcome, though thousands of early and provisional ballots remain to be counted across the district.
In a written statement, Kirkpatrick declared victory, thanked Paton and promised his supporters she would work to represent them in the U.S. House.
"There are few districts in our nation quite like CD1, and its diversity and complexities are not always understood by outsiders," she said. "Every day on the campaign trail, I was reminded of what unites the more than 80 communities across our sprawling district: creating jobs, protecting Medicare, strengthening education, helping our veterans."
Paton did not concede. Rather than speaking to reporters, he issued a written statement Wednesday afternoon laying out his position.
"I am deeply honored that so many Arizonans cast their ballots for our campaign," he said. "After a long night, our race is still too close to call. Currently, there are reports of tens of thousands of uncounted votes in Pima County alone. In our democracy, it is important that every legally cast vote is counted, and we will continue to monitor the results."
Late Tuesday, Paton was confident of victory, but it turned out that few ballots had been counted from the Navajo Nation, a Kirkpatrick stronghold. For her part, Kirkpatrick said she went to bed early Tuesday night and got eight hours of sleep, sure that the outcome was still uncertain.
"We knew all along after redistricting that it would be a competitive race," she said.
The new district encompasses about half of Arizona - 55,000 square miles - and a diverse landscape that includes 12 Indian reservations, bedroom communities in Southern Arizona, agricultural communities in Pinal County, mining towns in the Copper Basin, and tourism centers such as Sedona and the Grand Canyon in Northern Arizona.
The district includes all or part of nine of Arizona's 15 counties, making assessments of the race complicated. Thousands of ballots remain uncounted in Pima and Pinal counties, two of Paton's strongholds, but it was unclear how many of those are from the parts of those counties in Congressional District 1.
There are 11,000 uncounted provisional and early ballots in Coconino County, all of which is in the district, as well as thousands more in Navajo and Apache counties. All those areas voted strongly for Kirkpatrick.
For Democrats, winning the race means picking up a seat that Republicans previously held.
U.S. Rep. Paul Gosar, a Republican, beat Kirkpatrick, then the incumbent, in the 2010 election. After redistricting made the district more advantageous to Democrats, Gosar chose to move to Prescott and run in a more conservative district.
Contact reporter Tim Steller at 807-8427 or firstname.lastname@example.org