He alleged the BJP and RSS adhered to Hindutva ideology which is "inimical" to the basic values of the Constitution.
In an editorial in party organ 'Peoples' Democracy', Karat said, "The threat to the basic postulates of the Constitution emanates from the very people who are now entrusted with the task of upholding the Constitution and working its institutions.
"The ruling BJP-RSS combine adheres to the Hindutva ideology which is inimical to the basic values of the Constitution. The RSS and its ideologues have never hidden their contempt for the Constitution."
In the editorial, he made a reference to the remarks of Union minister Anant Kumar Hegde about changing it.
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Hegde had, however, said if someone was hurt by his remarks about the Constitution and about secularism, he had no hesitation in tendering his apology. He had also said his statement had been presented in a distorted manner.
Karat also said Uttar Pradesh Chief Minister Yogi Adityanath had "declared that the word 'secular' is the biggest lie told since Independence, thereby launching an assault on one of the basic principles of the Constitution".
He alleged, "Every institution under the Constitution, whether it be the judiciary, the civil service, or, the armed forces, are all being corroded from within and their integrity compromised."
He said, "These efforts would weaken the Constitution and erode it which constitutes an attack on the fundamental rights of citizens and poses a grave threat to the secular-democratic republic itself."
Karat said, "This assault had to be resisted and fought back. Only the people can do it as they are ultimately the custodians of the Constitution.