Mr Van der Bellen narrowly beat his 45-year-old opponent in May but an investigation revealed that there had been counting irregularities in several constituencies.
Millions of Austrians will head to the polls today in a presidential race that is still too close to call.
The race pits Norbert Hofer, of the far-right Freedom Party, against former Green Party leader Alexander Van der Bellen.
The two have vastly different views.
Mr Hofer is an engineer from a party that has capitalised on Eurosceptic and anti-immigration feeling in the country.
He has campaigned on promises to close Austria's borders to migrants and to "put Austria first".
Austria accepted 90,000 asylum seekers last year as the migration crisis unfolded and, while many people initially welcomed the newcomers, there has been hardening of attitudes since.
He is on record as saying it would be better if asylum seekers and immigrants "stayed at home".
"These people aren't working (in Austria) so I say give these asylum seekers the skills so they can rebuild in their own countries. Now that would be a meaningful task."
Mr Hofer's main base of support is in the country's more rural areas, where people think he will be good for the country's security and will do a better job of dealing with the migrant crisis.
If he were to win the election, he would be the first far-right leader in the European Union's history, a prospect that makes many of the bloc's leaders nervous.
Mr Van der Bellen, 72, is a retired economics professor who supports the EU, free trade and liberal policies towards migrants.
His support is mainly expected to come from Austria's urban areas but he is also seen as the political establishment, which may do him more harm than good in these new, Trumpian times.
He told his closing rally on Friday that Mr Hofer was trying to "demolish the house of Austria" instead of trying to repair it with "reason".
"We know that things need to change - but let's not destroy things," he said.