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 ±3.5 percentage points and ±3.4 percentage points among registered voters.