The Cook Political Report, a nonpartisan elections analyst, moved 21 House races in favor of the Democratic Party in its latest rankings released on Thursday.

The swing includes a mixture of safe and swing seats where Cook believes the Democratic Party has become more likely to either win a new seat or reelect an incumbent.

Dave Wasserman, a Cook analyst, pointed to strong Democratic fundraising in the final quarter of 2017 along with a consistent lead in the generic ballot as to why the group feels even more confident about Democratic changes in 2018. About 50 Democratic challengers out-raised Republican incumbents in the last three months of 2017, while only a small handful of Republican challengers raised more than the Democratic incumbent they hope to face.

Even though Republicans appear to be closing the gap on the generic ballot, which has shrunk to a 6-point Democratic lead, according to FiveThirtyEight's polling average, Wasserman said that shift isn't enough to make him optimistic about the GOP's chances.

"Most new district-by-district fundraising and polling numbers are downright terrible for Republicans, even in seats previously thought to be safe," Wasserman wrote in a blog post announcing the shift.

"The balance of evidence points towards a very wide - and mostly suburban - House battlefield with up to 75 GOP-held seats and fewer than 20 Democratic-held seats in play,"

Wasserman went on to describe Democrats as "ever-so-slight favorites" to secure the House majority after 2018, which would mean the party winning a net of 24 seats. And he went on to suggest that the Cook analysis "still understates Democrats' potential in individual races," arguing that more Republicans could be at risk if strong challengers emerge in those districts.

He also added that more GOP retirements or an upset victory by Democrats in a special election in Pennsylvania's red-leaning 18th District could give the party further enthusiasm ahead of November.

Cook moved four Republican-held districts to the "toss-up" category: the districts currently represented by Reps. Jeff Denham (Calif.), Mike Bost (Ill.), Erik Paulsen (Minn.) and Claudy Tenney (N.Y.). Five more districts - represented by Reps. Dave Brat (Va.), Scott Taylor (Va.), Dan Donovan (N.Y.), Ted Budd (N.C.) and Brian Mast (Fla.) - moved to "leaning Republican."

Seven Republican-held seats shifted from a "solid" to "likely" Republican.

Three open-seats previously rated as toss-ups - New Hampshire's 1st District, New Jersey's 2nd District and Pennsylvania's 7th District - are now rated as Democratic-leaning.

And thwo Democratic lawmakers - Reps. Stephanie Murphy (Fla.) and Kurt Schrader (Ore.) - were rated on more favorable grounds for Democrats.

Rep. Ben Ray Luján (D-N.M.), the chairman of the Democratic Congressional Campaign Committee, took a victory lap in a statement Thursday celebrating the announcement.

"The DCCC has continued to expand what is likely the largest battlefield in history," he said.

"We have a long way to go and won't take anything for granted, but are on track to take back the House in November."

Cook's analysis does not address the impact of the GOP's tax reform plan, which Republicans have pointed to as a potential saving grace ahead of the midterm elections. Democrats have taken advantage of the plan's low marks in its infancy to blast the plan as one that disproportionately benefits the country's top earners, and are expected to continue to do so.

But while more recent polling still shows the plan below majority approval, the tax plan's approval rating has increased in the first month of this year, giving Republicans some hope the party can point to the plan to help play defense at the ballot box.