Former Chancellor Kenneth Clarke has advised Tory leader David Cameron not to rely too heavily on soundbites.
Mr Cameron has grabbed headlines with his Commons performances
Mr Clarke, who was beaten to the Tory leadership by Mr Cameron, told GMTV he risked appearing unstatesmanlike.
Mr Cameron has grabbed headlines with his one-liners and put-downs of first Tony Blair and then Gordon Brown at question time in the Commons.
But in recent weeks Mr Brown has been accusing the Tory leader of using pre-scripted phrases.
During stormy exchanges at Wednesday's session, Mr Cameron said the prime minister was perpetrating a "con" on the public over Northern Rock, accusing him of "going from prudence to Del Boy without touching the ground".
Mr Brown accused Mr Cameron of "flip-flopping" on Northern Rock and of having nothing to offer but slogans.
Public 'not certain'
In an interview broadcast on GMTV's Sunday programme, Mr Clarke said Mr Cameron had to persuade the public to see him as a future prime minister by being "statesmanlike" in everything he does.
"He has the opportunity to be prime minister and the public are looking to him as a possible prime minister - that's a huge advance - but they're not certain yet, and because he's new, he's inexperienced, they're getting to know him," said Mr Clarke.
"I think he's capable of doing it myself. What he needs to approach everything with is a statesmanlike attitude of thinking, 'What am I going to do in government?'"
He added: "You need the odd soundbite, but not too many."
He said Mr Cameron faced a "long slog" until the next general election and had to show he was "a responsible person capable of governing the country - without being boring".
He added: "That should make him resist people who want him to make crazy promises, react to events, get a quick headline in one of the right-wing newspapers.
"He's got to calm his party down on the sillier things. He's got to lead his party responsibly into new areas, look as though he can govern a modern country."