ROME — As he leads the Five Star Movement in negotiations to form a new Italian government, Luigi Di Maio, 31 and brimming with confidence, seems to have a long political career ahead of him.

But according to the rules of his party, this is Mr. Di Maio’s last shot.

A fundamental rule of the Five Star Movement limits party members to two terms of elected office — at any level — over the course of their lifetime. Mr. Di Maio, who already served a term in Parliament, to which he was re-elected in March, is now in his second term.

The Five Star Movement, politically slippery and ideologically vague, has a record of bending its unbreakable party rules when victory is at stake, but for now, the term limit on Mr. Di Maio has added an element of now-or-never desperation to his bid to be Italy’s next prime minister.

It has also formed another complication in the stalemate after last month’s inconclusive election that may take weeks of negotiations to undo. Talks resume on Monday after a week in which the parties began testing the waters.