Former Vice President Dan Quayle has endorsed Mitt Romney for the Republican presidential nomination, calling him the only candidate who meets all the criteria for the job: leadership, character, conservative philosophy and electability.

There is only one candidate in the field that meets all of these criteria. It is Mitt Romney, Quayle wrote in an op-ed published in The Arizona Republic on Tuesday. He has proven over and over again that he is a leader. He has demonstrated he is capable of making tough decisions and turning things around. He is a man of integrity. He understands budgets and financial markets. He balanced budgets and met a bottom line. He is strong on national defense and has a deep love of the principles that make America great.

Quayle did not mention any other candidates by name, but some of his criteria included veiled jabs at Romney's opponents, and particularly at current front-runner Newt Gingrich.

We need a leader from outside of the Washington establishment, he wrote, as well as a person of integrity who has demonstrated that he can be trusted. Gingrich is the former speaker of the House and a known part of the Washington establishment, and he has a well-publicized history of adultery and ethics violations.

Quayle Says Romney Can Defeat Obama

Most importantly, Quayle wrote, Republicans need to nominate a candidate who can defeat Obama. That presumably disqualified the other five contenders -- Michele Bachmann, Jon Huntsman, Ron Paul, Rick Perry and Rick Santorum -- none of whom have polled well against Obama in hypothetical matchups.

Quayle served as vice president from 1989-1993 under President George H.W. Bush. He ran briefly for the Republican presidential nomination in 2000 but ultimately backed George W. Bush. His son, Ben Quayle, is a U.S. congressman from Arizona.

Quayle is the latest in a string of prominent politicians to endorse Romney. He joins New Jersey Gov. Chris Christie, who toyed with a presidential run himself; former Minnesota Gov. Tim Pawlenty, who dropped out of the presidential race earlier this year; and dozens of current and former members of Congress.

But while Romney has far more endorsements than any of his competitors, he has been unable to break 30 percent in national polls. He stands at about 20 percent now, while Gingrich is closing in on the 30 percent mark and rising.