Story highlights Despite a Republican victory, some GOP observers see warning signs in the special election

Republican state treasurer Ron Estes staved off Democratic and Libertarian opponents

(CNN) Republican Ron Estes beat back a surprisingly strong challenge from an unheralded and underfunded Democratic challenger to claim a special election victory in Kansas' 4th district on Tuesday night.

A win is a win -- and Republicans avoided the catastrophic outcome of losing in a congressional district where President Donald Trump won by 27 points last November. But in Estes' victory there are warning signs for Republicans preparing for the first midterm election of the Trump presidency in 2018.

"This should be a wakeup call to the Administration and the Republican Congress," said one Republican House member granted anonymity to offer an unvarnished view of what the special election meant. "The Democratic base is fully mobilized and unlikely to be defused. We will have to beat them. That will take motivating our base. So far we have not."

In Tuesday's special election, Democratic early vote was almost twice as large as that for Republicans, according to figures provided to me by a prominent Republican pollster. That's a sure sign of a base enthusiasm disparity. And in a district where there are typically twice as many registered Republicans as Democrats, early vote was 44.5% Democratic and 43.5% Republican.

Republicans appeared to do just enough in the final days to drive their voters to the polls; a visit to Wichita on Monday by Texas Sen. Ted Cruz as well as robo-calls from Trump and Vice President Mike Pence and a $100,000 ad buy by the House Republican campaign committee may have done just enough.

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