President Donald Trump with Vice President Mike Pence in the Blue Room during a 'Made in America' product showcase event at the White House in Washington, DC, on July 17, 2017.

To win back the Speaker's gavel for Nancy Pelosi , Democrats need a net gain of 24 seats in 2018. Wasserman sees a widening playing field in which the Democratic candidate has a chance in 61 GOP-held districts, compared to 20 Democratic-held seats where Republicans have a chance.

New ratings Wasserman published Friday alter the outlook of 12 individual House campaigns, in all regions of the country. Eleven of the 12 shifted to the benefit of Democrats.

"Democrats are on the cusp of where they need to be to take the House back," said the Cook Political Report's David Wasserman, who reviews House races district by district.

Midterm elections remain just more than a year away. But a leading nonpartisan analyst now sees a slightly better than even chance that Democrats win back the House in November 2018, which would halt Trump's current legislative agenda and even jeopardize his ability to complete his term.

While President Donald Trump takes a baseball bat to Obamacare and the Iran nuclear deal, odds are rising that he could break the Republican majority in Congress, too.

As in the 2010 midterm campaign during President Barack Obama's first term, the opposition party benefits from intense hostility to a controversial White House incumbent two years in office. Though Republicans have held onto four GOP districts with special elections this year, Democrats have displayed renewed energy by running closer in those districts than they have in the past.

"Everything we're seeking now is a mirror image of 2009," Wasserman observed. That includes widening support from the 10 percent share of the electorate he calls "check and balance voters" who seek to constrain the president.

The next 12 months give Republicans time to improve their fortunes. In particular, GOP leaders in Congress hope that success in passing tax-cut legislation will re-energize their own voter base after dispiriting failures to repeal Obamacare.

But Trump has been battling his own party as well as the Democrats. And he remains historically unpopular for a president this early in his tenure.

Gallup on Friday measured the share of Americans who approve Trump's performance at 39 percent, compared with 52 percent for Obama and 47 percent for Bill Clinton at the same points in their first terms. Both those Democratic presidents saw their party lose the House in their first midterm election.

President George W. Bush stood at 39 percent at the same point in 2005, during his second term. A year later, his fellow Republicans lost the House.

Because current Congressional district lines leave Democratic voters more concentrated in fewer districts, Wasserman figures Democrats need to win the national 2018 popular vote by 7 to 8 percent to gain a majority of seats. That's the size of their lead right now in the national polling average by Real Clear Politics, which shows 46.5 percent favoring Democrats and 38.7 percent favoring Republicans.

Democrats also face a tough 2018 battle to recapture the Senate, where Republicans now hold a 52-48 advantage and have fewer seats at risk. Of Senate seats on the ballot, the Cook Report gives Republicans a chance to win 13 held by Democrats, compared to four GOP-held seats where Democrats have a chance.

Given the power of the minority in the Senate, however, the consequences of a Democratic House takeover would be greater in any event. As Obama discovered after 2010, a hostile House majority can summarily roadblock a president's legislative priorities.

The potential for special counsel Robert Mueller's ongoing Russia investigation to find serious wrongdoing leaves Trump an even greater downside than that. The House holds the power of impeachment.