What have we here? Has a star been born? People are talking about that new television personality, Rep. Dan Rostenkowski of Illinois.

Yes, the gruff, square-faced machine politician from Chicago, Dick Daley's boy and wheeler-dealer chairman of the House Ways and Means Committee, made his debut responding to President Reagan's star-spangled, all-in-the family bid for tax equity -- and is hearing it for "Rosty" from both parties.

"Excellent," said the president's political director, Edward J. Rollins. "He is the only guy who has not paled beside the president in a Democratic response."

"He spoke right to the Knights of Columbus guy who voted for Reagan last time and might do it again, and he told him he's watching out for his interests," said an enthusiastic Chris Matthews, press secretary to House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.).

The calls and telegrams of support are pouring into the White House by the thousands, but Rostenkowski's office is getting them by the hundreds, which is like a street musician cutting into Michael Jackson's audience.

Rostenkowski, who has spent 27 of his 57 years in the House, is no more believable as a tax reformer than Reagan. In his first term, the president introduced the curious practice of letting big firms buy other firms' tax liabilities for purposes of deduction. Rostenkowski, during the epic battle over taxes in 1981, tried futilely to give more tax breaks to oil and gas interests than the Republicans gave them, a sordid exercise that sealed his reputation as a man who cares about victory, not substance.

But both the president and the congressman now see that tax reform is a consensus issue, and Tuesday night they set up rival cries for fairness and justice. The president cast his new enthusiasm in terms of protecting the family, starting a revolution and other familiar themes.

Both were powerfully motivated. After bad stumbles on his visit to Bitburg and defense spending, the president needed to resume the offensive. Rostenkowski wanted a way to advance his long-shot candidacy to succeed O'Neill.

Rostenkowski's job was especially delicate for a man who looks and talks like a stevedore. He had to make it clear, without whining, that the Democrats were first on tax reform. He had to welcome, without being smarmy or snide, the illustrious convert in the White House and to assure him of support where it is warranted.

The speech was drafted by Rostenkowski's press secretary, John Sherman. "The toughest part," Sherman said, "was to make him still a Democrat, while showing that to oppose the president's tax reform would be suicidal for the party."

Rostenkowski's ethnic background helped establish his Democratic identity. He was a meat-and-potatoes Democrat who grew up in a neighborhood and watched his friends move out to the suburbs. He talked in simple, declarative sentences about his Polish grandfather and, said one Democrat, "looked like someone right out of 'Cheers.' "

"I hardly recognized him," said a colleague, "being so gracious to the president. He shouts a lot around here."

For his all-important match with the champ, Rostenkowski got himself a coach. At the suggestion of Democratic Congressional Campaign Committee Chairman Tony Coelho of California, he called on Joe Rothstein, a savvy and seasoned political consultant.

"Being from Chicago, he has never had to use TV. Democrats have to think of it as a telephone, an essential tool of communication," said Rothstein.

Rostenkowski submitted to unfamiliar disciplines for the camera. He put in contact lenses. He agreed to smile. They rehearsed the speech. Tone, they decided, was everything.

Rostenkowski's team briefly considered incorporating some reply to Reagan's out-of-the blue denunciation of the Democrats the day before. In Florida, Reagan blasted them for being soft on communism and dividing the country. They decided that despite his call for bipartisanship on tax reform, Reagan might be trying to provoke them into an attack on the program that would get them in dutch with voters. Rothstein and Sherman decided against taking the bait. Rostenkowski had to prove he was a Democrat, not a partisan.

When Rostenkowski was through, the phone started ringing in the committee room. Several state chairmen wanted to pay their respects to a man who at 8:15 Tuesday night was a hack and who, 15 minutes later, had become TV's political rookie of the year.