President Barack Obama has built a seven point lead over Republican rival Mitt Romney in Ohio, a key bellwether state, and has a five point advantage in Florida and Virginia, a poll said.

Those states are key to winning the election but Romney has failed to gain ground in all three areas in the two weeks since the Republican National Convention in Tampa, Florida, said the Wall Street Journal/NBC/Marist survey.

Obama leads Romney 50 percent to 43 percent in Ohio, a major coal producer and US industrial heartland whose voters tend to pick election winners, the poll said, and 49 percent to 44 percent in Florida and Virginia.

The president's poll lead comes despite continued turbulence in the US economy, with only 96,000 new jobs being added in August and with thousands of unemployed Americans abandoning their job search altogether.

Although the uncertain economic outlook has dominated the argument on the campaign trail, Romney's repeated attacks on Obama's fiscal competence have consistently failed to gain traction in opinion surveys.

Obama won Ohio, Florida and Virginia in 2008, but four years earlier the then Republican president George W. Bush was returned to the White House after sweeping all three states.

It is thought that Romney would need to win at least two of the three on November 6 if he is to reach the magic mark of 270 electoral college votes that are needed to capture the presidency, according to pollsters.

However, given the states that are already seen as being solidly locked up for Democratic candidate Obama, it is thought the president could be re-elected even if he only wins one of the three states.