Republican presidential candidate Herman Cain has vaulted to the top of several key polls this week. (REUTERS)

Republican Herman Cain’s presidential campaign is on the rise — and so are the political attacks against him.

A top adviser to GOP rival Mitt Romney unloaded on Cain’s “9-9-9” tax plan yesterday as polls show businessman Cain leading or tied with the former Massachusetts governor in several national and state polls.

That’s a sure sign that Romney, a favorite of the GOP establishment, now considers Tea Party darling Cain a serious threat.

A new Rasmussen poll showed Cain and Romney tied with 29 percent of the vote.

And a survey in the battleground state of Florida, released by the American Research Group yesterday, found Cain ahead of Romney, 34-28.

A Wall Street Journal/NBC survey Wednesday had Cain garnering 27 percent of the vote to 23 percent for Romney. Texas Gov. Rick Perry captured 16 percent and Ron Paul 11 percent.

With Cain surging, Romney spokesman Eric Fehrnstrom took aim at Cain’s “9-9-9” plan, which scraps the current tax code and calls for a 9 percent flat tax on corporate income and personal income and a new 9 percent national sales tax on all purchases.

“I don’t think the American people are ready for a national sales tax. The people of New Hampshire are not ready for a national sales tax,’’ Fehrnstrom told Fox News Channel yesterday.

For his part, Cain, the former CEO of Godfather’s Pizza who seemed to be relishing the increased scrutiny and attention, took some populist barbs of his own at Romney while stumping in Cincinnati.

“He’s more of a Wall Street guy. I’m more of a Main Street guy,’’ Cain said of Romney, who co-founded and ran a venture-capital firm before entering politics.

“I actually made some pizza myself,’’ Cain said.

Cain said he’s connecting with GOP voters “because of my enthusiasm and positive attitude.”

Meanwhile, Cain drew plaudits from key Republicans yesterday.

Mississippi Gov. Haley Barbour on the Laura Ingraham radio show called Cain a “straight talker’’ and a “very, very attractive’’ candidate.

“If Herman Cain is our nominee against Barack Obama,’’ Barbour said, “I think he’ll [Cain] sweep the South.”

Wisconsin Rep. Paul Ryan, considered a leading GOP policy wonk, gave Cain’s “9-9-9” plan a thumbs up.

“We need more bold ideas like this because it’s specific and credible. I’m a flat-tax kind of guy,’’ Ryan said.

Meanwhile, President Obama campaign reported raising $70 million over the past three months.