NEW DELHI - Two powerful parties refused today to join Sonia Gandhi in a coalition government, setting off the biggest plunge in the 129-year history of India's stock market and twice forcing regulators to halt trading.

Investors feared that Gandhi would have to backtrack on her pledge to go forward on the liberalization of the economy.

More than $45 billion in market capital has been wiped out since share prices began tumbling last Thursday, when it became clear Gandhi had ousted the Hindu-nationalist coalition led by Atal Bihari Vajpayee in national elections.

As she prepared to seek formal approval from the president for the formation of what will now have to be a minority government, Hindu nationalists added to the turmoil with street protests against Gandhi's bid to become the nation's first foreign-born leader.

The communists said they would back Gandhi's ascension as prime minister in coming days, but not from within the coalition ranks.

Trading had to be halted for the first time in the history of the Bombay Stock Exchange, after it plunged a record 15 percent within 90 minutes.