The impeachment inquiry of U.S. President Donald Trump could turn into a big risk for House Democrats, especially if it gets drawn out, experts told CNBC.

Democrats should try to go through the process as quickly as possible, as it could spark sympathy for Trump if it drags on, said Larry Sabato, founder and director of the University of Virginia Center for Politics.

On Tuesday, House Speaker Nancy Pelosi announced that the U.S. House of Representatives will begin an official impeachment inquiry into Trump over his efforts to push Ukraine to investigate a potential 2020 election rival, former Vice President Joe Biden.

"The Democrats will be wise to act expeditiously. Ideally they would have this wrapped up before the voting begins," Sabato said, referring to primaries in February ahead of the general election.

"Because the longer this is stretched out ... you're going to generate some sympathy for Donald Trump in the long run," Sabato told CNBC on Friday. "That'll do it if they stretch this out for months and months and months."