The current LP Pledge is required by section 5.1 of the bylaws:
Members of the Party shall be those persons who have certified in writing that they oppose the initiation of force to achieve political or social goals.
Some extremists in the LP try to use the Pledge to castigate members who disagree with anarcholibertarianism, but this distorts the original meaning of the Pledge. LP founder David Nolan created the Pledge in 1971 to protect the party from possible accusations that the LP seeks violent overthrow of the U.S. government. But there is now no prospect of a COINTELPRO-style government threat to the party. When many (perhaps most) moderates took a pledge in joining the Libertarian Party, they were joining a party that they believe had competently named itself and thus had consciously decided not to call itself the Anarchist Party. As they signed up to be a party activist and support the party's political actions, it didn't seem very strange that the party might ask them to pledge tactical non-violence so as to give the party plausible deniability for anything destructive that the new members might try to do in its name. The oppressively legalistic atmosphere of the modern nanny state makes such silly CYA certifications all too common.
Many of them were aware of how anarchists use the phrase "initiation of force", but it was always in the context of absolute abstinence for any purpose. The LP Pledge qualifies the usual anarchist formula with the above vague language about "political or social goals". If the LP had intended an oath of absolute fealty to the Zero Aggression Principle, it would have used a normal and unqualified statement of it. One could reasonably conclude that these "political or social goals" must be a reference to the goals involved in the step they as Pledgers were taking: adopting the Party's goals as their own.