Republicans are gaining ground on Democrats ahead of the November midterm elections, according to a new poll that suggests the battle for control of Congress could be tighter than many political observers expected.

The latest Economist/YouGov survey, released on Tuesday, found Democrats holding only a narrow lead over Republicans on the congressional generic ballot, a significant shift from earlier polling this year.

Among registered voters, Democrats led Republicans by just 46% to 44%, respectively. Among all adults surveyed, Democrats held a 39% to 36% advantage over Republicans.

The results mark continued movement in the GOP's favor. An Economist/YouGov poll in February showed Democrats with a 7-point lead on the generic ballot. By May, that advantage had narrowed to 5 points.

The latest numbers could strengthen Republican hopes of holding onto their House majority despite historical trends that often favor the party out of power during midterm elections.

The Economist/YouGov findings are not the only warning sign for Democrats.

A Harvard-HarrisX poll found the electorate evenly divided, with 50% of registered voters saying they would support their local Democrat candidate and 50% backing the Republican candidate.

While Democrats maintained a 52% to 48% edge among independents, Republicans appeared to hold a slight advantage when it came to voter enthusiasm.

Overall, 71% of respondents said they were "definitely" or "probably" going to vote. Among Republicans, 62% said they would definitely vote, and another 18% said they would probably vote.

For Democrats, 59% said they would definitely vote, and 17% said they would probably vote. Among independents, 40% said they would definitely vote, and 17% said they would probably cast a ballot.

The polling comes as both parties ramp up efforts to energize voters ahead of what is expected to be a fiercely contested fight for control of Congress.

The Economist/YouGov poll was conducted June 13-15 and surveyed 1,549 U.S. adults, including 1,403 registered voters. The margin of error for the entire sample was plus or minus 3.5 percentage points and plus or minus 3.4 percentage points among registered voters.