The CPI(M) today decided to commit itself to an alliance with the ‘‘Congress-as-the-opposition’’ for the Andhra polls to defeat the BJP-TDP combine after having backed out in the other states as the Congress is the ruling party there.‘‘We will extend support to candidates of the Left and secular parties, including Congress, who can defeat the TDP-BJP in areas where we are not contesting. We will mobilise all forces to defeat the TDP-BJP combine,’’ senior Politburo member Prakash Karat said.Though the CPI has not made its stand official, senior leaders of the party said the need to adopt ‘‘suitable tactics’’ to rout the BJP-TDP coalition has made an alliance with Congress the most prudent option.The CPI(M) is measuring the Andhra elections with the same yardstick as the Gujarat polls — the only two occasions on which it decided to set apart its traditional anti-Congressism.‘‘Our view on the matter is quite clear: The BJP-led NDA nexus has to be defeated. Andhra will be a good test for our alliance,’’ a senior Left leader said. ‘‘It’s a political battle and our fight is against the BJP,” said Karat, justifying the party decision.