Mikhail Gorbachev is now the ''second-secretary'' of the Soviet Communist Party Politburo and is the ''probable'' candidate to succeed Konstantin Chernenko as the leader of Soviet Union.
Mr. Gorbachev, however, has not been given sole responsiblity for party ideology. Mr. Chernenko is ''for the time being'' doing ''most of the work in ideology himself.''
These comments, from a senior party official, confirm that 53-year-old Mikhail Gorbachev - the youngest member of the ruling Politburo - is now effectively the second-in-command in the Kremlin hierarchy. They also indicate, however, that he has not yet won for himself full responsibility for the ideology portfolio - thought to be essential for anyone who aspires to become a Soviet leader.
Many Western Kremlin-watchers have been working under these assumptions, but official confirmation has not been forthcoming until now.
The official, who has detailed knowledge of the inner workings of the party power structure, said in an interview that ''Gorbachev has occupied, as it were, the (position of) second-secretary of the party . . . practically, and as a matter of fact.''
Gorbachev, an agricultural specialist, has risen rapidly since attaining full Politburo membership in 1980. Now, he seems well-positioned - though not assured - to rise even higher in the leadership.
Though the Soviet official spoke of Gorbachev as the probable next general secretary of the party, he cautioned against the assumption that Gorbachev would automatically be elevated.
In fact, being No. 2 in this country is by no means a guarantee of being No. 1. Nor, it appears, is even the personal endorsement of the sitting general secretary. It is believed Leonid Brezhnev favored Konstantin Chernenko as his successor. The post was claimed insteadby Yuri Andropov.
Similarly, analysts say Andropov hand-picked Gorbachev to succeed him; Chernenko, however, edged Gorbachev out.
Still, it appears that when Andropov died in February, Gorbachev had amassed enough power to broker second place for himself.
One key indicator of how successfully he continues to consolidate his power will be whether he eventually takes over the ideology portfolio, thus becoming the figurative flame-keeper of Marxist-Leninist doctrine. The post has traditionally been a powerful one. Its most influential occupant in recent times was Mikhail Suslov, who until his death in 1982, was described as the undisputed ''kingmaker'' of the party.
Now, according to the party official, Gorbachev is chairing meetings of the party Secretariat - composed of the secretaries that head the party's professional staff departments - as well as continuing to oversee economic reforms implemented by Yuri Andropov.
The official adds that Gorbachev is expanding his interests into foreign affairs. The official also denied suggestions of disarray or indecisiveness in the Kremlin under Chernenko.