In a letter dated 26 June, the group said Prime Minister Viktor Orbán's government was systematically removing checks and balances to its powers.
There was no one immediately available to comment from Hungary's government or foreign ministry.
The writers, philosophers and other people who opposed Hungary's old communist regime said the new centre-right government had restricted the freedom of the press, curbed the powers of the top court and passed a new constitution earlier this year which tied the hands of future governments (see 'Background').
"An autocratic system is in the making in Hungary," they wrote to Clinton and the US Ambassador in Hungary.
"The historic visit of President George Bush in 1989 helped us Hungarians to establish democracy in our country. Your visit may help us to prevent its demolition today," they added.
During his visit to Budapest in July 1989, former US President George Bush gave a speech in front of the parliament building to thousands of Hungarians, saluting the reforms and changes that were taking place at the time.
Clinton is due to arrive in Budapest today (28 June) and is scheduled to meet Hungarian Foreign Minister János Martonyi and the prime minister on Thursday.
The intellectuals included writer György Konrád and former OSCE Representative on Freedom of the Media Miklós Haraszti.
They said the government – an alliance between the centre-right Fidesz and KDNP parties that won a two-thirds parliamentary majority in 2010 – was threatening democracy.
"[Orbán's] ruling coalition systematically demolishes the constitutional guarantees of separation of powers, removing all checks and balances that restrain the executive," they said.
The intellectuals said the media authority created under the new media law this year was a one-party authority that could deny media outlets the renewal of their licences.
Hungary's media law came under sharp criticism within the EU after it was passed in 2010.
In April the government also passed a new constitution which critics said would cement its powers further and tie the hands of future governments in key areas of policy.
EurActiv with Reuters
