Estes defeated Democrat Jim Thompson, a civil rights attorney and Army veteran, and Libertarian Chris Rockhold.
The seat was considered to be by Republicans a straight win -- until an energized Democratic base left Estes with a smaller advantage.
Trump tweeted about the race Tuesday morning, urging voters to get out and support Estes over his Democratic challenger.
"Ron Estes is running TODAY for Congress in the Great State of Kansas. A wonderful guy, I need his help on Healthcare & Tax Cuts (Reform)," Trump tweeted
On Monday, Estes had another influential GOP name stumping for him -- Texas Sen. Ted Cruz. Cruz, who won the state in the 2016 Republican primaries, introduced Estes at a rally and called on district residents to turn out to vote for the Republican.
"Today the eyes of the whole country are upon Kansas," Cruz said at the rally. "This election, this special election tomorrow, makes a difference."
Trump, as well as Vice President Mike Pence, recorded a robocall endorsing the candidate for the seat
. House Speaker Paul Ryan sent out a fundraising plea to Republican donors on behalf of Estes, the Kansas City Star reported.
Also, the National Republican Congressional Committee spent nearly $100,000 on ads in the closing days.
Trump congratulated Estes on Wednesday: "Great win in Kansas last night for Ron Estes, easily winning the Congressional race against the Dems, who spent heavily & predicted victory!"
The district is solidly Republican -- Trump won the district in the general election by 27 points and 2012 Republican nominee Mitt Romney won the district by eight points. A Democrat hasn't represented the district since Dan Glickman was ousted in the 1994 Republican wave election.
Meanwhile, the Democratic Campaign Committee has not spent money to help Thompson at all -- and the Kansas State Democratic Party rejected his requests for funding mailers.