US President Donald Trump gestures to supporters as he arrives for a campaign rally in Rio Rancho, New Mexico, on September 16, 2019.Nicholas Kamm | AFP | Getty ImagesThe 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.”In fact, Trump seemed to have already cashed in on the development. His re-election campaign raised a quarter of a million dollars in just 15 minutes on Tuesday in the immediate aftermath of the announcement about the probe.On Thursday evening, Eric Trump — Donald Trump’s son — tweeted that the campaign has raised $8.5 million in the last 48 hours since the announcement. “You are handing @therealDonaldTrump the win in 2020!” He tweeted.Peter Trubowitz, professor of international relations at the London School of Economics, said the whole affair is “definitely a big risk” for the Democrats.”People forget that the Republicans in 1998 lost the House after impeaching Bill Clinton,” he told CNBC.During the 1998 midterm elections amid the impeachment proceedings led by the Republicans, the Democrats actually gained seats. Republican House Speaker Newt Gingrich stepped down, while Clinton proceeded to finish his second term.— Reuters contributed to this report.