Washington (CNN) The generic congressional ballot has continued to tighten, according to a new CNN poll conducted by SSRS, with the Democrats' edge over Republicans within the poll's margin of sampling error for the first time this cycle.

About six months out from Election Day, 47% of registered voters say they back the Democratic candidate in their district, 44% back the Republican. Voters also are divided almost evenly over whether the country would be better off with the Democrats in control of Congress (31%) or with the GOP in charge (30%). A sizable 34% -- including nearly half of independent voters (48%) -- say it doesn't matter which party controls Congress.

The Democrats' advantage in the generic ballot dipped from 16 points in February to six points in March to just three points now. The party's advantage has waned among enthusiastic voters as Republican enthusiasm has grown (in March, 36% of Republican and Republican-leaning registered voters said they were very enthusiastic about voting; that's up to 44% in the new poll), but the Democrats still have a double-digit lead among those most excited to vote this fall (53% of those who are very enthusiastic about voting say they'd back the Democrat in their district vs. 41% who say they favor the GOP candidate). Those enthusiastic voters also say by a 10-point margin that the nation would be better off with Democrats in control of Congress than Republicans.

By 48% to 43%, registered voters say they would rather back a candidate who opposes Donald Trump than one who supports the President. That margin has narrowed from the 52% who opposed Trump to the 41% who supported him in January.

Read More