Southern Arizona could deliver the state's biggest surprise of the election.
Arizona's three competitive congressional races are still too close to call, including the Tucson-centered seat Rep. Ron Barber won in June to replace his former boss, Gabrielle Giffords.
Barber saw his small lead against Republican Martha McSally in the 2nd District disappear just before midnight, with McSally taking a tiny lead.
In a statement, Barber's campaign said there were reports of voting problems in Pima County and that many ballots remained to be counted.
Barber added that his campaign will continue to watch the results of the race.
In the new Phoenix-area 9th District, Democrat Kyrsten Sinema and Republican Vernon Parker are in a near dead heat.
Both Parker and Sinema acknowledged the wait would be long, and Parker urged his supporters to be patient because thousands of ballots remained uncounted.
Sinema's camp said Wednesday afternoon it was optimistic her lead would expand and would make sure "every vote is counted."
In northeastern Arizona's 1st District, Republican Jonathan Paton's slight lead over Democrat Ann Kirkpatrick diminished as Tuesday night wore on.