Washington (CNN) President Donald Trump recorded a robo-call for Ron Estes, a Republican running for an open House seat in a special election in Kansas' 4th district.

The Kansas Republican Party tweeted , "Donald Trump recorded voter turnout call in support of Ron Estes going out to tens of thousands in KS-04."

Donald Trump recorded voter turnout call in support of Ron Estes going out to tens of thousands in KS-04. #ksgop #ks04

Vice President Mike Pence also recorded a robo-call for Estes, the Kansas state treasurer, and House Speaker Paul Ryan sent out a fundraising plea to Republican donors on behalf of Estes, The Kansas City Star reported.

And the National Republican Congressional Committee is spending nearly $100,000 on ads in the closing days.

The seat, which belonged to Mike Pompeo before he was tapped by Trump to be CIA director, was considered by Republicans to be a straight win -- until an energized Democratic base left Estes with a smaller advantage.

Estes is competing for the seat against Democrat Jim Thompson, a civil rights attorney and army veteran, and Libertarian Chris Rockhold.

On Monday, conservative Sen. Ted Cruz of Texas stumped for Estes in Wichita, calling on residents to vote for him.