In 2016, Republicans controlled the house of representatives with 241 seats versus democrat's 194. No third party candidates won any seats. Republicans have controlled the house since 2010, under the Obama administration. Historically, there is often a backlash against the winner of the presidential election during their first term, and current polling on a national generic ballot indicates that Democrats have an 6-8 percentage point lead over Republicans.
If Democrats win 218 seats or more in 2018, they will have secured a majority in the house. Will that happen?
435 seats will be contested on November 6th, except for Louisiana, whose general election occurs on December 8th. The question resolves after enough races have been called by reliable media reports to give control to one party or another. In case of ongoing ambiguity, the question can resolve when the Congress convenes in January 2019 and elects a Speaker of the House.