As seen in this answer Dasratha invited the assembly to debate his decision to coronate Rama and presented the pros and cons of Rama's candidature with a neutral viewpoint.
But as per verse 35 of the 110 Ayodhya kanda sarga, Vasishta says
इक्ष्वाकूणाम् हि सर्वेषाम् राजा भवति पूर्वजः |
पूर्वजेन अवरः पुत्रो ज्येष्ठो राज्ये अभिषिच्यते || २-११०-३५
The eldest son only becomes the king in the entire Ikshvaku race. When the eldest son exists, the younger son will not become a king. The eldest son only is anointed to the crown.
Now how does one reconcile the above two incidents. If eldest son was to become King in Ikshvaku then why did Dasratha go on to explain pros and cons of Rama being the heir ?
On a related note, if it was given that eldest son was to be crowned king then Manthara would not have waited till the Dasratha's announcement to poison Kaikeyi's mind.