Obtained by NBC News

DES MOINES, Iowa -- When Iowans opened their mailboxes this week, they found a lot of campaign literature criticizing none other than the current front-runner, Newt Gingrich.

With just days to go before the Iowa caucuses, at least five separate direct-mail pieces from the Mitt Romney, Ron Paul and Rick Perry campaigns are floating around the Hawkeye State targeting the former House speaker. In addition, a pro-Romney SuperPAC, Restore Our Future, also has anti-Gingrich literature out.

Newt Gingrich is the “latest flavor of the month,” a letter from Ron Paul to his fellow conservatives says. “This particular flavor, however, will leave a bitter taste, as there are serious problems with his policies and beliefs.”

Obtained by NBC News

Romney’s campaign literature links Gingrich to fellow former House Speaker Nancy Pelosi. It shows a distorted black and white version of a TV ad that the two former speakers did together and reads: “When Al Gore needed allies … he turned to his friend Nancy Pelosi to push job-killing climate change legislation. When Pelosi wasn’t enough … They turned to Newt Gingrich.”

Restore Our Future also tries to link Gingrich with Pelosi. “Pelosi and Gingrich: More in common than you think,” a mailer from the PAC says. It accuses Gingrich of co-sponsoring 418 bills with her in Congress and of flip-flopping on a number of issues, including global warming. “When you attend the January 3^rd caucus, ask yourself: Who is the consistent conservative who can defeat President Obama? IT’S NOT NEWT GINGRICH,” the double-sided mailer reads.

In another mailer paid for by Restore Our Future, it asks “Why does the Obama machine want to run against Newt Gingrich?” and lists 11 different issues that it says Gingrich has “flip-flopped on” over the years. The front side of the mailer depicts a smiling President Barack Obama holding a newspaper with a faux headline reading “Newt wins Iowa.”

And Texas Gov. Rick Perry took on both Gingrich and Romney in his new mailer, questioning whether conservatives can trust either of the front-runners with their vote. “Mitt Romney and Newt Gingrich have abandoned our values and profited at our expense,” the ad reads. “Romney and Gingrich: supported Obamacare mandate. Wrong on abortion. Rich off of America’s misery.”

Obtained by NBC News

These anti-Gingrich pieces are just the latest in a slew of attacks on him from his GOP rivals, including attacks on the air in Iowa.

In just a 30-minute period during Friday's nightly news program on a local television channel in the Des Moines market, eight presidential campaign ads ran during commercial breaks. Half the ads that ran were attacking Gingrich (paid for by Restore Our Future, Ron Paul or Rick Perry). The other four were positive ads by the Paul, Romney and Perry campaigns plus one from a pro-Rick Santorum SuperPAC.

Gingrich vows to remain positive in his campaign for the nomination and only hit one opponent: Barack Obama.

Obtained by NBC News

Obtained by NBC News