Incumbent Democratic U.S. Sen. Mark R. Warner defeated Republican challenger Daniel M. Gade to retain the seat he has held since 2009.

“I’m incredibly proud and grateful to the people of the Commonwealth for sending me back to work,” Sen. Warner said during a news conference Tuesday night.

“While we’re all waiting on the national results, I’m optimistic that this is going to be a very good night for our country.”

The Associated Press called the race for Sen. Warner early Tuesday night, even as the initial count of ballots cast earlier in the day placed Mr. Gade in the lead.

However, Sen. Warner continued to close the gap throughout the night, with unofficial results reported by the state Department of Elections on Wednesday showing Sen. Warner with more than 2.3 million votes, or 55.68 percent, compared to 1.8 million for Mr. Gade.

Mr. Gade, a retired Army lieutenant colonel from Northern Virginia who teaches at American University, initially refused to concede the race. In a speech Tuesday evening, he criticized AP for projecting that Sen. Warner would win re-election. But on Wednesday morning, Mr. Gade acknowledged losing the contest.

“I’m deeply grateful to the tens of thousands of donors and volunteers who powered this campaign and the millions of Virginians who honored me with their vote last night,” Mr. Gade said. “I concede and call on all of my supporters to recognize state and federal results with peace and unity.”

In his remarks Tuesday night, Sen. Warner pointed to a string of successes by Virginia Democratic candidates in the past few years as positive change and cited the potential for a Democratic majority in the Senate once the final tallies come in races across the country.

There’s “a lot of work to do, and I look forward to getting back to it,” Sen. Warner said. “Brighter days are ahead for both Virginia and our country.”

However, his forecast for Senate change did not appear to be holding up. Thirty-five of the 100 Senate seats were up for election, but only 14 were considered seriously competitive.

Democrats, whose caucus currently includes 47 members, needed to win at least three seats to gain control if Sen. Kamala Harris of California is elected vice president. As vice president, she would become the Senate’s presiding officer in January and the official tie breaker.

With races still undecided Wednesday evening, Democratic challengers ousted Republican incumbents in Arizona and Colorado, but took a step backward when a Republican challenger in Alabama defeated Democratic incumbent, Sen. Doug Jones.

Republican incumbents also won top Senate races in Iowa, Maine, Montana and South Carolina and Kentucky, where Senate Majority Leader Mitch McConnell fended of a challenge from Democrat Amy McGrath.

Analysts now consider it more likely than not that Sen. McConnell will remain in charge as majority leader when the results are in for all the races, including Georgia, where there could be two runoff elections to determine winners of that state’s two U.S. Senate seats.

Staff writer Jeremy M. Lazarus contributed to this report.