With the anti-immigration party topping the poll in the first round last Sunday, Hollande's spokesman Stephane Le Foll quoted the president as saying there "needed to be clarity in the behaviour and attitude of all political leaders to defend the values of the Republic".
He said the elections were particularly important given the power that regional governments have over education, transport and environment policy.
"The regions have a major role in deciding the future" of the country, he said.
French presidents usually try to be seen as remaining above party politics.