Is Herman Cain a legitimate candidate or just the flavor of the day for Republicans?

And just like that, after months of referring to former Massachusetts Gov. Mitt Romney as the front-runner for the Republican presidential nomination, that title — at least for the moment — got passed on to Texas Gov. Rick Perry yesterday.

And it wasn’t just that Perry inched past Romney, who as of July topped the GOP leader board with 23 percent support among Republicans and Republican-leaning independents in Gallup’s national poll. Perry blew right by him, surging to 29 percent compared to Romney’s 17 percent. Trailing behind are Ron Paul (13 percent) and Michele Bachmann (10 percent).

Other candidates, including Herman Cain, Newt Gingrich, Rick Santorum and Jon Huntsman were polling in the low single digits, according to Gallup’s latest numbers. Add possible candidates Sarah Palin and Rudy Giuliani to the mix and Perry still comes out on top with 25 percent, followed by Romney (14 percent), Palin (11 percent), Paul (11 percent) and Giuliani (9 percent). And it’s not just the Gallup poll.

As The New York Times’ Nate Silver points out, Perry has taken the lead over Romney in a total of five national polls conducted since he officially jumped into the presidential race earlier this month. The front-runner mantle is not necessarily something that the Perry campaign was hoping for at this stage in the race, and there’s no telling for how long it will stick. Indeed, a Perry source told ABC News yesterday that “the only poll that matters is on election day.”

Keep in mind, however, that the percentage of Republicans who say they are undecided remains essentially unchanged from last month. It was 20 percent in July and 19 percent this month. In other words, it’s still fair to call the situation in the GOP primary “fluid.”

Perry’s lead comes not by convincing those sitting on the sidelines to join him, but by essentially stealing votes from Romney and Bachmann. In theory, these voters could always move back to their original candidate if they decide that Perry doesn’t wear well. It’s also important to remember where things stood in the GOP primary at this point in 2007.

Remember those days? Former New York Mayor Rudy Giuliani was the favorite to capture the Republican nomination, leading the field with 34 percent support as of early Sept. 2007.

Following on his heels was former Sen. Fred Thompson at 22 percent, Sen. John McCain at 15 percent, and Romney at 10 percent.

We all know how that turned out — Giuliani’s campaign ended after he limped to a third-place finish in the 2008 Florida primary, Thompson’s campaign fizzled even earlier (after South Carolina) and McCain captured the nomination. At this point in 2003, Sen. Joe Lieberman was in the lead among Democratic voters with 23 percent, followed by Congressman Dick Gephardt (13 percent), former Vermont Governor Howard Dean (12 percent), and the eventual nominee — Senator John Kerry (10 percent).

And, four years later, in the fall of 2007, Hillary Clinton held a double-digit lead over Barack Obama from the summer through early fall. Rick Perry’s biggest challenge now will be the Sept. 7 debate at the Reagan Library in California where he will have to show a command of policy issues.

He's great on the stump, but how will he do at the podium? ON TODAY’S “TOP LINE. ABC’s Amy Walter and Rick Klein will be joined by Rob Jesmer, Executive Director of the National Republican Senatorial Committee and Guy Cecil, Executive Director of the Democratic Senatorial Campaign Committee to talk about the political landscape for both parties heading into 2012.

Watch “Top Line” LIVE at 12:00 p.m.