Duff told this website, "Nobody should doubt that Verhofstadt's real ambition is to succeed Jean-Claude Juncker as Commission President."
MEPs will elect the new President for the next two and a half years in a secret ballot on Tuesday in Strasbourg.
If not elected by an absolute majority of valid votes cast during the first three ballots, MEPs will choose the new President from the two best scoring candidates of the third ballot, by a simple majority, in a fourth round.
The election starts on Tuesday morning with short presentations by each of the candidates and might last until late in the evening.
Parliament's outgoing President Martin Schulz will chair the opening of the plenary session on Monday evening and announce the candidates for the first ballot in the election of his successor.
Schulz will also chair the election process and the new President will take up his or her post immediately after the election.
Duff, a constitutional expert, said, "On the face of it, as things stand, Antonio Tajani is the favourite.
"But, if elected, all three EU institutions would be then led by the EPP. There is much to be said against such a monopoly on top posts.
"Tajani's election would surely weaken the case for the automatic re-appointment of Donald Tusk to the presidency of the European Council in May - a re-appointment which is in any case hotly and rudely opposed by the Polish government."
Duff says that if Verhofstadt wins he would be in a "strong" position to lead the Liberal campaign at the 2019 elections to the European Parliament - just as Schulz did as the top candidate for the Socialists in 2014.
But Duff believes Verhofstadt's "real ambition" is to succeed Juncker. He had hoped several years ago to land the Commission job but was knocked back by the then-UK Prime Minister Tony Blair, who considered him too federalist for such a post.
Duff said, "Chairing the Parliament judiciously for a couple of years would be the best possible way to dispel his reputation in Council circles as a firebrand.
"By June 2019, Brexit will have removed the habitual British veto of any Belgian federalist who vies for the top Commission job."
Duff expects a "substantial abstention" in the later ballots from the forces of the traditional far-right, "most of which loathe the EPP."
In considering the key elements of the election, he says, "Nationality does not really matter in this context - at least for the vast majority of MEPs who easily fit into Europe's classic partisan spectrum.
"Indeed, it would be counter-intuitive to favour a candidate who was a fellow-countryman but from a rival political camp.
"And with three Italian candidates, being Italian will help none of them. A sense of 'fair turns' would scupper the chances of another German from winning the crown (even among Germans MEPs).
"In certain circumstances, MEPs could be influenced by regional instinct to vote across party boundaries - for example, by supporting a fellow Scandinavian or central European - but the absence of either breed from the contest this time is striking."
Duff, now a visiting fellow at the European Policy Centre, added, "Similarly, gender could and should be a factor, but with the three women candidates destined to come low in the political ranking, it cannot be so this time."
He pointed out, "There is no black or ethnic minority candidate.
"So the predominant influence will be that of party affiliation, in spite of the fact that group cohesion, especially in the EPP and ALDE is unreliable.
"In a house whose raison d'être is the European project, degrees of support for further European integration is a significant factor.
"Strong nationalists will never vote for strong federalists, and vice versa. But as no nationalist candidate can win, it is the mildly 'pro-European' Tajani, in contradistinction to Pittella or Verhofstadt, who can pick up Eurosceptic votes in later rounds."