With Trump suffering from the fallout of yet more controversy, Hollande said, "Democracy is at stake as we see more and more people tempted by authoritarianism, especially in the United States.
"Should the American people choose Trump, there will be consequences, because a US election is a global election," he added.
His intervention comes after the pan European centre-right European People's Party said it was reluctant to support Trump in the upcoming presidential race because of his rejection of strong ties with the EU.
Its failure to back a Republican candidate marks a significant break from convention.
The EPP says its support for a given US candidate will depend on their intentions about future transatlantic relations, including on the economy, security and immigration.
Meanwhile, the S&D group in Parliament has thrown its weight behind Democratic candidate, Hillary Clinton.
An S&D delegation led by group leader, Italian MEP Gianni Pittella, and Vice-Chairs Knut Fleckenstein and Tanja Fajon, was in Philadelphia to attend the Democratic convention. It met with several US personalities, among others, Bernie Sanders, Bill Clinton, Madeleine Albright and the trade unions President, Richard Trumka.
Speaking of Clinton, Pittella said, "She is widely experienced, strongly motivated, a promoter of human and minority rights, a progressive who believes in improving the life chances of everyone in society.
"We welcome the proposals that strongly reinforce fiscal and social justice and fight tax evasion, in particular on how to widen health and child-care rights, how to secure equal access to education throughout all social classes, how to guarantee equal pay for equal work, gender equality, women and minorities' rights."
Pittella added, "This is not a normal election between a Democrat and a Republican candidate. There is much more here at stake: US democracy, peace and stability in the world and transatlantic relations.
"The US has been infected by the same virus of populism, racism, xenophobia that has affected Europe. This virus in Europe is called Le Pen in France, Farage in Great Britain, Orbán in Hungary, Northern League in Italy and Kaczynski in Poland. In the US this virus is called Donald Trump.
"Business as usual is not an option, neither for Europe nor the US. The world needs a reliable, effective and democratic Europe and US. The world surely does not need Trump or his populist friends in Europe. Donald Trump is just not reliable.
"The US and EU must work together to tackle terrorism, poverty and war, migration and other global challenges."
"The world and the EU need the United States of America. We need Hilary Clinton to become the next US President."