The opposition Move Forward and Pheu Thai parties dominated last week's election in a resounding defeat of conservative parties backed by a royalist military that has controlled government since a 2014 coup.
They are seeking to form a coalition government with six other parties, all of which are due to sign the agreement on their objectives later on Monday. Reuters was able to review a draft of the pact.
The signing takes place on the ninth anniversary of a military coup against a democratically elected government that ushered in phase of military rule that the winners of the May 14 polls hope to end.
Move Forward was the surprise election winner, emerging with the most seats with the help of young voters excited by an agenda that puts the party at odds with some conservative big business interests and institutions, including a plan to amend a lese-majeste law that punishes perceived insults of the monarchy with long jail sentences.
Other alliance members have had reservations about tampering with that law.
Asked about the preliminary list of proposals drawn up by Move Forward and circulated among all prospective coalition members, senior Move Forward official Bencha Saengchantra said alliance members were in agreement on 80% to 90% of the plans "but it can still be adjusted".
The party had on Friday said lese-majeste would be included only if the eight parties backed it. Bencha reiterated Move Forward would seek amendments in parliament independently.
The draft includes most of Move Forward's flagship polices, such as a push for decentralization of power and budget and to "cancel monopolies and support fair competition in trade in all industries".
It also takes aim at the military, calling for ending mandatory conscription and reform of the armed forces as well as the justice system and civil service, "corresponding with democratic principles with emphasis on transparency, up-to-date, effectiveness and prioritising the public interest".
Thailand’s military has staged 13 coups since the end of absolute monarchy in 1932 and confrontation between the generals and civilian politicians and activists has been at the heart of recurring instability over the years.
The alliance will also seek reform of welfare and education, a balanced foreign policy and pursuit of laws to both control and endorse the use of cannabis, which Thailand legalised last year, despite confusion over regulations.
Move Forward leader Pita Limjaroenrat is seeking to be prime minister at the head of the coalition but faces a big challenge in winning the required support from more than half the combined lower and upper houses, which includes rivals with whom his party has clashed.
His alliance comprises 313 seats, but it needs backing from 376 legislators to vote Pita in. He will likely need to win over some of the 250 members of the conservative-leaning Senate, which was appointed by a junta and has often sided with army-backed parties.