Vermont Sen. Bernie Sanders is giving the Hillary Clinton campaign more to worry about than just strong poll results in the first two nominating states in the election process. He’s also raising massive amounts of money and showing he’s able to compete with what is generally seen as one of the strongest fundraising operations politics ever has seen.

Sanders raised $20 million in January from more than 770,000 individual donors, the campaign confirmed to the Wall Street Journal, a haul that is nearly two-thirds the amount the senator raised in the final quarter of 2015. Contrasting with Clinton’s fundraising portfolio, just 1.3 percent of Sanders’ donors have reached the maximum $2,700 limit the Federal Election Commission imposes on campaign donations.

“Working Americans chipping in a few dollars each month are not only challenging but beating the greatest fundraising machine ever assembled,” Jeff Weaver, the campaign manager for Sanders, said Sunday in a statement.

Clinton, according to campaign finance data compiled by the Center for Responsive Politics, has raised more money than any other candidate in the 2016 race so far with $77.5 million given to her campaign’s committee. Sanders, though, has proved surprisingly adept at raising cash and has some advantage over Clinton since most of his donors can be tapped again for more donations in coming months. His total at the end of last year was $41.5 million.

The two will face off Monday in Iowa, where voters will have a chance for the first real time to register their preferences for the Democratic nominee. Clinton has a slight lead over Sanders in the state, according to averages of polls compiled by Real Clear Politics. She leads with 47.3 of the vote, compared to her opponent’s 44 percent. The third Democrat in the race, former Maryland Gov. Martin O’Malley, pulls in just 4.4 percent of the state’s vote.