Democrats hold an advantage ahead of the midterm elections, but a Washington Post-ABC News poll shows that edge has narrowed since January, a signal to party leaders and strategists that they could be premature in anticipating a huge wave of victories in November.

The poll finds that the gap between support for Democratic vs. Republican House candidates has dropped by more than half since the beginning of the year. At the same time, there has been a slight increase in President Trump’s approval rating, although it remains low. Measures of partisan enthusiasm paint a more mixed picture of the electorate in comparison with signs of Democratic intensity displayed in many recent special elections.

One potentially new factor in the mix of midterm issues is gun policy, which has emerged as a major voter consideration two months after the mass shooting at Marjory Stoneman Douglas High School in Parkland, Fla. More than 4 in 10 registered voters say it is extremely important that ­candidates share their views on gun issues. Fewer voters say it is critical that candidates share their views on Trump or House Minority Leader Nancy Pelosi (D-Calif.), leaders who are most likely to be targets in partisan messaging this fall.

[Read full poll results | How the poll was conducted]

With the Republicans’ House majority at risk, 47 percent of registered voters say they prefer the Democratic candidate in their district, while 43 percent favor the Republican. That four-point margin compares with a 12-point advantage Democrats held in January. Among a broader group of voting-age adults, the Democrats’ margin is 10 points, 50 percent to 40 percent.

Republicans owe part of their improved standing to Trump’s thawing job ratings. The Post-ABC poll finds that 40 percent approve of the president, up slightly from 36 percent in January to his highest level of support since last April. Still, Trump continues to face majority disapproval at 56 percent, higher than any other president at this stage since the dawn of modern polling, an indication that he remains a significant liability for Republicans on ballots in November.

The survey shows the GOP making a more pronounced shift among white voters, who now prefer Republicans by a 14-point margin over Democrats, up from five points in January. Republicans lead by 60 percent to 31 percent among white voters without college degrees, slightly larger than an 18-point GOP advantage three months ago.

The situation in the districts where control of the House is likely to be decided is slightly more favorable for Democrats. The Cook Political Report, which produces nonpartisan analysis, lists 56 of the 435 congressional districts as competitive — 51 of them in Republican hands to just five held by Democrats.

In competitive districts excluding Pennsylvania, where new boundaries were drawn this year, Democrats have an edge of 50 percent to 43 percent when voters are asked which party’s candidates they would favor if the election in their district were held today. Democrats need a net gain of 23 seats to capture the majority in the House.

Special elections and gubernatorial races over the past year have shown that Democrats are benefiting from a surge in voter enthusiasm, including a narrow victory in Pennsylvania’s 18th Congressional District in March, which Trump won by nearly 20 points in 2016.

The Post-ABC poll finds parity in stated voting intentions. Among registered voters, 68 percent of both Republican-leaning and Democratic-leaning registered voters say they are certain they will vote. This contrasts with Post-ABC polling ahead of the 2010 and 2014 midterm cycles, when Republicans averaged a double-digit advantage in intentions to vote. Democrats suffered major losses in both years.

Other public polls have found a narrowing in Democrats’ midterm advantage, although it has been less sharp than in the Post-ABC poll.

An average of public polls compiled by The Post finds Democrats’ lead on this metric stood at eight points in January and 11 points in February but six points in polls over the past 30 days, similar to the Post-ABC poll’s four-point margin. Analysts expect Democrats to need a six- to eight-point lead in “generic-
ballot polls” to win a majority of congressional districts.

The new survey points to opportunities and challenges for both parties in coming months.

Some core constituencies of each party expressed tepid interest in turning out to vote in an off-year election, when many eligible voters typically stay home. Although 58 percent of all adults say they are sure they will vote this year, that falls to fewer than 4 in 10 among adults younger than 30. Young voters have heavily favored Democrats in recent elections. Certainty to vote dips to 54 percent among African Americans and 39 percent among Hispanics. Those compare with 64 percent among whites, a majority of whom favor Republicans.

At the same time, white voters with college degrees, a competitive voting bloc, are 14 points more likely to say they are certain to vote than whites with some college or less, a group that has increasingly favored Republicans and voted for Trump at record levels.

Sixty-one percent of men and 56 percent of women say they are certain to vote, with 55 percent of female registered voters saying they favor a Democratic candidate and 50 percent of men backing a Republican. Democrats are counting on strong turnout among women to help their candidates in November.

The renewed gun-control debate is a wild card in the midterm election, with lawmakers facing pressure from students nationwide to pass new laws. Several polls have shown heightened support for restrictions aimed at curbing gun violence following February’s massacre in Parkland.

Although public activism has put pressure on Republicans and the National Rifle Association, the Post-ABC poll suggests that neither party holds an advantage in support among the 42 percent of voters who say it’s “extremely important” that a congressional candidate share their views on the issue. Within this group, three-quarters of voters who prioritize enacting new gun laws support Democrats for Congress, while 8 in 10 of those who give protecting gun rights greater significance support Republicans. As a whole, the group splits nearly evenly, with 47 percent supporting Democrats and 46 percent backing Republicans.

A smaller group, 31 percent, say it is “extremely important” for congressional candidates to share their views about Trump, although more than half say this will be at least “very important.” Those who say it is extremely important favor Democrats over Republicans by 11 points, 54 percent to 43 percent.

Many Republicans are trying to make Democratic leader Pelosi a focus of their campaigns. In the poll, 17 percent of voters say a candidate’s views on Pelosi will be extremely important in their vote, and Republicans lead Democrats by 16 points among this group in the generic congressional ballot.

Pelosi has a negative image, with 32 percent of Americans holding a favorable view of her, and 44 percent unfavorable. But nearly one-quarter have no opinion of the former House speaker, who could regain the gavel if Democrats flip the House. Among Republicans, she is well-known and widely disliked, with 74 percent holding unfavorable views of her, 63 percent strongly.

The Post-ABC poll was conducted April 8-11 among a random national sample of 1,002 adults reached on cellphones and landline telephones. Overall results have a margin of sampling error of plus or minus 3.5 percentage points; the error margin is four points among the sample of 865 registered voters.

Emily Guskin contributed to this report.