Showing a bounce in his step, the three-term Texas governor jogged onto the stage at the Western Republican Leadership Conference and delivered a 15-minute speech vowing to roll out an economic plan next week that would scrap the current tax code and replace it with a flat tax.

The plan was announced in the wake of the much-touted 9-9-9 tax overhaul plan that has sent Herman Cain in some polls to the top of the GOP field.

“I want to make the tax code so simple that even Timothy Geithner can file his taxes on time,” Mr. Perry said, alluding to the tax problems President Obama’s Treasury secretary acknowledged before his confirmation in 2009.

Mr. Perry also picked up where he left off in the testy debate the previous night, implicitly questioning Mr. Romney’s conservative credentials, as he worked to revive some of the ideological concerns that helped derail the former Massachusetts governor’s 2008 bid.

“A ‘change election’ requires a new direction and not more of the same — and I come by my conservatism very authentically, not by convenience,” Mr. Perry said, assuring the gathering of Western conservatives that he is not the establishment’s pick nor a flip-flopper on the issues that are important to them.

In a sign of the growing heat between the two contenders, the Romney camp struck back, releasing a one-minute “Ready To Lead?” Web video highlighting some of Mr. Perry’s worst moments in recent debates, where he stumbled over his words, meandered through a response to a question about the nation’s policy toward China and refused to offer details of his economic plan.

The Romney forces also launched a website, careerpolitician.com, returning to a line of attack that the former Massachusetts governor employed earlier this year, contrasting the time he spent in the private sector against Mr. Perry’s more than 25 years working in government.

After entering the race in August and shooting to the front of the GOP field in national polls, Mr. Perry’s star fizzled after a series of poorly reviewed debate performances. His GOP rivals attacked his support of in-state tuition for some illegal immigrants and the executive order he signed requiring girls at public schools to be vaccinated for the sexually transmitted human papillomavirus, which is known to cause cervical cancer.

Mr. Perry’s fall opened the door for Mr. Cain, an Atlanta talk-show host and former Godfather’s Pizza CEO, to rise in the polls on the back of his call to replace the federal tax code with his signature 9-9-9 plan, which entails scrapping the complex federal tax code in favor of a 9 percent national sales tax and a 9 percent tax on business and personal income.

Mr. Cain also has been on the defensive about his proposal.

His campaign also lacks the fundraising and ground operations that could be needed to perform well in the early primary states, leaving some political observers to predict that his top-tier status will be short-lived — especially given the more than $14 million war chest that Mr. Romney and Mr. Perry reported in their third-quarter fundraising reports.

But in a speech here Wednesday, Mr. Cain showed he’s still a favorite of grass-roots Republicans, generating much more excitement than Mr. Perry, while defending his tax plan.

“The American people get it. It’s the ones in Washington, D.C., who have a vested interest in the current tax code who don’t want 9-9-9 to succeed,” he said, suggesting that the “lobbyist brains have exploded” over Mr. Cain’s push to strip all the loopholes out of the federal code.

Hours earlier, Mr. Perry had entered the tax policy debate in earnest with his call for a flat tax, which remains popular in a lot of conservative circles.

The announcement put him on a much different ground than Mr. Romney, who rolled out an economic plan in September that calls for less dramatic change to the federal tax code. The Romney plan proposes cutting the top corporate income-tax rate from 35 percent to 25 percent, advocates for lower individual rates over “the long run” and eliminates the capital-gains tax for people who earn less than $200,000 a year.

Perry supporters say the governor may have started to right his ship Tuesday night when he badgered Mr. Romney over his support of universal health care in Massachusetts, his record on job creation and his policies on immigration. The Texan accused Mr. Romney of knowingly hiring illegal immigrants to work at his Boston home, a charge that Mr. Romney said was false, but a line of attack that had the former Bay State governor looking uncharacteristically agitated.

“The idea that you stand here before us and talk about that you’re strong on immigration is, on its face, the height of hypocrisy,” Mr. Perry said, sparking a mix of boos and cheers from the crowd.

Joan Huffman, the Texas state senator who introduced Mr. Perry to the crowd here Wednesday, told The Washington Times that while the merits of the immigration attack are debatable, it is clear that Mr. Perry is growing more comfortable with the debate format.

“We are starting to see the Perry we know,” Ms. Huffman said. “I think he is ready to get in there now and engage.”

The debate fireworks reflect in part the calculation among Republicans that given President Obama’s poor approval ratings, the Republican who emerges as the party’s nominee will have a solid chance of becoming the next president.

In his speech Wednesday, Mr. Perry drove home the point that the GOP horse race is far from over.

“If the pundits and the establishment think they choose our president, primary and caucus voters haven’t gotten the memo,” Mr. Perry said. “The American people are not going to trim around the edges in 2012; they are going to turn Washington inside out. I am not the candidate of the establishment. You won’t hear a lot of shape-shifting nuance from me.”