Washington, July 18 (IANS): Democratic Party's presumptive presidential nominee Hillary Clinton is maintaining a distinct advantage over Republican rival Donald Trump headed into the two upcoming national party conventions this month, according to three national opinion polls.

The surveys released on Sunday all showed Clinton with a single-digit lead, from 4 to 7 points, Politico reported.

An ABC News/Washington Post poll gave Clinton a 4-point advantage among registered voters, 47 per cent to 43 per cent. 

Two per cent of voters said they preferred another candidate, six per cent would vote for neither Clinton nor Trump, and two per cent offered no opinion which is much closer than the former Secretary of State's 12-point lead in June -- though that was one of her best polls since both locked down their parties' respective nominations, the poll revealed.

Clinton's lead was larger in a CNN/ORC International poll: 7 points, 49 per cent to 42 per cent. 

Four per cent volunteered that they would vote for another candidate, four per cent would not vote for either candidate and one per cent had no opinion, Politico reported citing the poll.

Unlike the ABC News/Washington Post poll, the CNN/ORC survey represented a slight increase for Clinton from its mid-June poll, which showed her leading by 5 points.

The NBC News/Wall Street Journal poll largely splits the difference: Clinton leads by 5 points, 46 per cent to 41 per cent, with a significant percentage, eight per cent, saying they preferred neither candidate. Another five per cent picked another candidate or were undecided.

That was unchanged from the June NBC News/Wall Street Journal poll, which also showed Clinton with a 5-point lead.

The ABC News/Washington Post poll was conducted from July 11-14, surveying 816 registered voters with a margin of error of plus or minus 4 percentage points. 

The CNN/ORC poll was conducted July 13-16, surveying 872 registered voters with a margin of error of plus or minus 3.5 percentage points. 

The NBC News/Wall Street Journal poll was conducted July 9-13, surveying 1,000 registered voters. It had a margin of sampling error of plus or minus 3.1 percentage points.

The Republican National Convention will take place from Monday to Wednesday in Cleveland, Ohio, while the Democratic National Convention is scheduled to take place from July 25 to 28 in Pennsylvania.