Conventional wisdom is that Democrats will take control of the House and Republicans will hold onto their Senate majority in Tuesday's elections, but there is a small probability that won't be the case, top election analyst Larry Sabato told CNBC.

"There is a small chance, like the chance of pulling an inside straight in poker, which you can do but which doesn't happen very often … that the Republicans will hold the House," said Sabato, director of the University of Virginia's Center for Politics and the editor in chief of Sabato's Crystal Ball.

"And there's an inside straight chance that Democrats will get 51 seats, which is the minimum they need to control the Senate," he said on "Power Lunch."

An inside straight is an incomplete poker hand that has four of the five cards needed to get a straight but one of the middle cards is missing.

Sabato's nonpartisan Crystal Ball, which relies on things such as private polling, public polling and social media analysis, is predicting Democrats will pick up 34 House seats. The minimum they need to gain for the majority is 23. It sees Republicans holding onto the Senate or maybe even adding a seat to bring their total to 52.

In the final forecast on Monday, Sabato and Managing Editor Kyle Kondik wrote, "We have long cautioned against assuming the House was a done deal for the Democrats, and we don't think readers should be stunned if things go haywire for Democrats tomorrow night."

However, they added that it may be just as likely, or even more likely, that they are underestimating the Democrats in the House.

Last week, Kondik and Sabato pointed out that midterms often lead to a decline for the presidential party after a surge in the House in the presidential year. However, the GOP didn't enjoy a surge in 2016 and therefore it can limit its decline this year.

For the Senate, if Democrats win every race where they were currently favored, plus Indiana and Missouri, and one in North Dakota, Tennessee or Texas it is "possible" they can gain control, according to the latest Crystal Ball.