The lead held by Democrats over Republicans on generic ballot polls ahead of the 2018 midterm elections is beginning to slip, a new CNN poll suggests.

Overall, 31 percent of respondents in a poll released Wednesday told CNN that they believe the country would be better off with Democrats in control of Congress, while 30 percent said Republicans should hold the reins. However, the largest proportion of respondents, at 34 percent, said it makes no difference to them who is in charge.

Among registered voters asked whether they would vote Democratic or Republican in their congressional district if the elections were held today, Democrats had a three-point advantage, at 44 percent to 41 percent, which is within the poll's margin of error.

Democrats have seen a steady decline in their advantage over Republicans in recent months, according to CNN polling, falling from a 16-point advantage in February to a 6-point one in March, to just a 3-point lead this week, roughly six months away from the midterm elections.

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An ABC News/Washington Post poll similarly found last month that Democrats' lead over Republicans among registered voters was only 4 points, at 47 percent to 43 percent, down from a 12-point lead the poll found Democrats held in January.

Democrats still have an edge in enthusiasm, according to CNN. Among respondents who said they are excited to vote in November, more plan to vote Democratic than Republican, at 53 percent to 41 percent.

But enthusiasm does seem to be growing among GOP voters. According to the CNN poll, 44 percent of Republican and Republican-leaning registered voters said they were "very enthusiastic" about voting, which is a jump from 36 percent in March.

The 2018 midterms will decide whether Republicans will maintain control of both chambers of Congress.

The CNN/SSRS poll surveyed 1,015 adults from May 2-5 and had a margin of error of plus or minus 3.6 percentage points overall, and a 3.8 percentage point margin of error among the 901 registered voters surveyed.