Fifty-one per cent of polled voters said that they would vote for Biden over Trump, while only 41 per cent said that they would vote for Trump.
Biden had an 11-point lead in the same poll conducted the month before.
Half of the NBC News and Wall Street Journal poll was conducted after Senator Kamala Harris was announced as Biden’s running mate and the vice presidential nominee.
It is a long-standing tradition for the party’s nominee for president to announce his or her running mate before the party convention.
The four-day Democratic national convention begins on Monday and is set to largely take place online. (dpa/NAN)