WASHINGTON — Many of Mitt Romney’s presidential challengers are having trouble fulfilling a fundamental requirement of running for public office: getting on the ballot.

Rick Santorum, Newt Gingrich, Rick Perry and Jon Huntsman have all failed to qualify for the ballot in at least one upcoming GOP primary. In other states, they have failed to file full slates of delegates with state or party officials, raising questions about whether these candidates have the resources to wage effective national campaigns.

And if one of them were able to marshal enough anti-Romney forces to challenge the front-runner, the ballot blunders could limit their ability to win delegates in key states.

The exception: Ron Paul, who appears to have avoided such pitfalls so far.

“This is why you need a real-life, no-kidding-around campaign,” said Rich Galen, a GOP strategist and former Gingrich aide who is neutral in the 2012 race. “All these guys who have been crowing that they found a new way to run for president, it’s like saying I’m inventing a new airplane, and it has only one wing.”

Romney, the former Massachusetts governor, won the first two contests, in Iowa and New Hampshire, and he is leading in the polls in South Carolina and Florida, the next two states to have primaries. Romney raised $56 million in 2011 for his campaign, giving him big financial and organizational advantages over his GOP rivals.

Those advantages are on display as many of his competitors miss deadlines or fail to collect enough signatures to meet ballot requirements in upcoming contests.

Santorum, the former Pennsylvania senator who came within a few votes of winning the Iowa caucuses, didn’t get on the ballot in Virginia or the District of Columbia. His campaign also filed incomplete slates of delegates in Illinois and Ohio, which could limit his ability to win delegates in those key states.

Virginia has been a tough ballot to crack for several GOP candidates because the state requires campaigns to collect signatures from at least 10,000 registered voters. Romney and Paul were the only ones who made the ballot for the March 6 primary. Perry sued, and he has since been joined in the lawsuit by Gingrich, Huntsman and Santorum.

Santorum is the only major candidate who will be left off the ballot in the District of Columbia primary April 3, said Paul Craney, executive director of the DC Republican Committee. The party provides two ways to get on the ballot: Pay $10,000, or pay $5,000 and collect signatures from 296 registered Republicans in the heavily Democratic capital city.

“It’s not easy, but it can be done, if you are a serious presidential candidate,” Craney said. “All the presidential candidates who are serious about winning the nomination will be on the D.C. ballot.”

Santorum adviser John Brabender acknowledged that Romney has more money and a larger campaign organization. But, he said, Santorum’s campaign has gained resources and momentum since the close finish in Iowa. Romney, he said, has been running for president for the past six years, giving him more time to build his organization.

“It’s a different campaign than it was earlier,” Brabender said.

Huntsman, the former Utah governor, failed to get on the ballot in Arizona or Illinois.

The requirements to get on the GOP ballot in Arizona are pretty easy — all you have to do is fill out a two-page form. Twenty-three candidates managed to do it properly, so they will be on the ballot for the state’s Feb. 28 primary.

Huntsman, however, was left off the ballot because his filing had a photocopied signature and wasn’t notarized, said Arizona Secretary of State Ken Bennett.

“If you can’t get on the ballot around the country, you’re a regional candidate, by definition,” said Rich Beeson, the Romney campaign’s political director. “Barack Obama is going to be on the ballot in all 50 states, and so will we.”

Gingrich, the former House speaker, didn’t make it on the ballot for primaries in Missouri or Virginia, though he has joined the lawsuit to get on the Virginia ballot and Missouri won’t award any delegates based on its Feb. 7 primary. Instead, Missouri Republicans will hold caucuses March 17.

Perry, the Texas governor, made the ballot in Illinois, but he will only be eligible to win one delegate in the state’s March 20 primary — a contest in which 54 delegates will be up for grabs.

It will take 1,144 delegates to win the nomination at the Republican national convention this summer.

Illinois has a unique way of awarding delegates to candidates. The winner of the state’s GOP primary doesn’t necessarily get any delegates. Instead, Republicans will vote for the actual delegates, who are listed separately on the ballot but are identified by the candidate they support.

Each of the state’s 18 congressional districts will elect three delegates, for a total of 54. To appear on the ballot as a delegate, candidates had to collect signatures from at least 600 registered voters in the district where they are running.

Only one Perry delegate filed signatures by the deadline, according to the Illinois State Board of Elections. Gingrich, Paul and Romney filed full slates, while only 44 Santorum delegates filed signatures.

Ron Michaelson, who served as executive director of the Illinois State Board of Elections for nearly 30 years, said he doesn’t remember presidential candidates having these kinds of problems in previous elections.

“They’re concentrating so heavily on the early states, devoting so many resources there that they’re not looking down the road far enough,” said Michaelson, now a visiting assistant professor at the University of Illinois in Springfield.

The system in Illinois is designed to keep fly-by-night candidates from crowding the ballot, said Christopher Mooney, another political science professor at the University of Illinois.

“It keeps people who don’t know what they’re doing out of the arena,” Mooney said.