Democratic candidates have outperformed expectations in every special election since President Donald Trump took office, and analysts are starting to worry that the GOP could lose the House and the Senate in 2018.

Trump has already struggled to push his agenda through Congress, and losing his slim majorities in both chambers could stall his already slow-moving agenda.

A narrow win in deep red Arizona on Tuesday night highlights how real and close this danger is for Republicans.

Democratic candidates have outperformed expectations in every special election since President Donald Trump came into office, and it's increasingly looking as if Trump's 2018 nightmare of losing both the House and the Senate in midterm elections could come true.

Republicans only narrowly squeaked out a win in an Arizona special election on Tuesday night, with their candidate winning by 5 points in a state Trump carried by 21 points.

A report at The New York Times found Republicans losing a similar amount of ground in popular support in other key states Trump won in 2016, including Georgia, South Carolina, Alabama, and Pennsylvania.

"Even the reddest of districts in a red state can be in play this year," the Associated Press wrote in an article analyzing Tuesday's slim win for the GOP in Arizona.

Steven Law, who runs American Crossroads, a well-funded group supporting Republican Senate campaigns, told the news website Axios it was "not likely but not out of the question" for the GOP to lose the Senate in 2018.

"We do have more defensive terrain to hold than when the cycle started," Law said, "and targeted Democratic incumbents have been over-performing in terms of their early fundraising activity."

Republicans hold the Senate by the slimmest majority possible, at 51 to 49 seats, and losing even one or two seats could further hamper their ability to enact Trump's already slow-moving agenda.