As the old saying goes, politics ain’t beanbag.

So why, then, are Newt Gingrich and Rick Perry coming under such strong fire from members of their own party for their criticism of Mitt Romney’s record at Bain?

President Obama and former president Bill Clinton at the White House. (J. Scott Applewhite/Associated Press)

Of course, it was then-Sen. Hillary Clinton (D-N.Y.), not Bill, who was running for president in 2008. But the former president was doing more than his share of stumping on his wife’s behalf in the hard-fought Democratic primary.

And it was Bill Clinton’s attacks on then-Sen. Barack Obama (D-Ill.) during the heated intra-party battle that drew sharp rebukes from senior Democrats – much as Perry and Gingrich find their party establishment closing ranks against them today.

What distinguished Bill Clinton’s attacks from those of others in the Democratic primary battle?

Two things – one of which is relevant to the current GOP primary fight, the other not:

1) Some of Clinton’s criticism of Obama echoed attacks that Republican candidates, not fellow Democrats, would be expected to make against a Democratic White House hopeful.

In a December 2007 interview with Charlie Rose, for instance, Clinton said that to elect Obama would be to “roll the dice” when it comes to national security. At the time, Obama and Hillary Clinton had been battling fiercely over their national security résumés – but Bill Clinton’s remark, which drew headlines far and wide, was viewed as tantamount to a declaration that Obama was unfit to serve as commander-in-chief.

That’s the kind of intra-party attack that can do long-lasting damage to a candidate once the General Election rolls around, a reality of which fellow Democrats at the time were keenly aware.

And it’s the same category of attack that Gingrich and Perry are now making against Romney, arguing that the former Massachusetts governor is a “vulture capitalist” who “looted” companies without regard for the jobs losses that might result. Those remarks are exactly in tune with the income-inequality argument that Democrats and the White House have been making against Republicans for months – and are expected to double down on once the general election hits.

Criticism of Romney’s tenure as governor might be considered fair game by other members of his party, but attacks on his record in venture capital – much like Bill Clinton’s “roll the dice” remark – are most certainly viewed as crossing an important line.

2) In criticizing Obama, Bill Clinton at times veered into identity politics, at times subtly and at others, not so. This type of attack has not reared its head in the 2012 GOP primary to date.

In 2008, Clinton drew harsh rebukes when, at an event in New Hampshire, he called Obama’s campaign narrative on Iraq “the biggest fairy tale I’ve ever seen” — a remark interpreted by some Democrats as dismissing the candidacy of an African American presidential hopeful.

And two weeks later, as Obama was poised for a big win in South Carolina, Bill Clinton noted that “Jesse Jackson won South Carolina in ’84 and ’88.”

That comment was roundly criticized by senior Democrats as dismissing Obama’s success in South Carolina as a function of race: Rep. James Clyburn (D-S.C.), who had warned Clinton in the run-up to the Palmetto State’s primary to “chill a little bit,” denounced the remark, and the alleged injection of race by Clinton into the campaign reportedly contributed to Sen. Ted Kennedy’s (D-Mass.) decision to endorse Obama over Hillary Clinton.

Might any candidate or outside group make this type of attack in the current Republican primary fight? If there’s anywhere it might happen, it would likely be South Carolina, where GOP presidential battles have a history of turning ugly.

But Bill Clinton’s experience provides a cautionary tale – voters as well as party elites often view such attacks as having no place in politics. And just as Clinton’s popularity took a (temporary) hit after his comments in the 2008 race, any candidate seen as making identity-charged attacks this time around could also find himself in hot water with voters.