A Colorado voter told Denver7 that someone forged her signature on a petition to place Republican U.S. Senate candidate Jon Keyser on the ballot for the June primary election.

Pamela Niemczyk of Littleton told Denver7's Marshall Zelinger on Tuesday that she had signed a petition for Jack Graham, another Republican U.S. Senate candidate. She said the signature on the Keyser petition was not hers, calling it a "fraud."

"It's fraud, it's definitely fraud. I'm not happy with it," said Niemczyk. "It looks like they put an 'R' on the end. And it looks like they didn't even, when they spelled it out, didn't spell it right."

ProgressNow Colorado said in a news conference on Tuesday that it initially discovered "several invalid petition signatures" -- including two signatures attributed to Niemczyk on different candidates' petitions -- while conducting a "cursory investigation of the Secretary of State’s own data" on campaign petitions.

The left-leaning advocacy group said this "revealed several invalid petition signatures that evidently slipped through the cracks, and at least one wrongly accepted duplicate signature that appears to be fraudulent."

"Our review of just a sample of Jon Keyser’s petitions in one congressional district has found enough uncaught invalid signatures to raise serious questions about whether Jon Keyser has in fact qualified for the Republican primary ballot," said ProgressNow Colorado political director Alan Franklin. "The Secretary of State must immediately rescind their statement of sufficiency for Jon Keyser, and request an extension of the temporary injunction now in place to revisit errors and potential fraud in the signature validation process. 'Close enough' isn’t good enough for Colorado voters."

Secretary of State spokeswoman Lynn Bartels said her agency was checking to see how the bogus signature got past officials and if they have identified any other fraudulent signatures.

Several of the candidates hired professional signature-gathering firms, which is a routine practice in politics.

The Keyser campaign issued a statement blasting U.S. Sen. Michael Bennet, the Democratic incumbent whom he and other Republican candidates are vying to challenge in the November general election.

"Senator Michael Bennet’s liberal friends, ProgressNow Colorado, embarrassed his campaign yet again today with a flailing stunt that clearly telegraphs to the entire political world how scared Michael Bennet is to face Jon Keyser in November," Keyser spokesman Matt Connelly said in the statement. "…The entire political world knows Jon Keyser will be on the ballot and we appreciate Progress Now's invitation to highlight for conservatives across the country that Jon Keyser is Senator Bennet's worst nightmare."

Whether or not it was political stunt, Denver7 confirmed that the Littleton voter said someone forged her signature on the Keyser petition.

"I don't think this is a liberal stunt, I think it's backpedalling by the Keyser group to cover their tracks. They got caught and they're trying to cover it now," said Niemczyk. "I don't know how anyone could trust you after this."

The controversy comes after several Republican U.S. Senate candidates' have struggled to get their names on the ballot because the Secretary of State's Office found some of their petition signatures were invalid.

Last month, Secretary of State Wayne Williams, a fellow Republican, said that Keyser had fallen 86 signatures short in Congressional District Three, in western and southern Colorado. Williams disqualified 186 signatures collected by Tyler Gonzalez. The Secretary of State could not confirm the address Gonzalez was listed as a registered Republican, with the address that he was currently living.

But a Denver district court judge ruled last week that Keyser can be on the primary ballot.

"Mr. Gonzalez’s failure to update his voter registration data with his new address is de minimus, especially given the fact that his move occurred just days before he started gathering signatures for Mr. Keyser," District Judge Elizabeth Starrs wrote in her ruling. "In assessing Mr. Gonzalez’s credibility and the evidence in this case, the Court finds that Mr. Gonzalez did not have an improper motive in failing to update his voter registration address. The Colorado Election Laws are designed, in part, to eliminate fraud. No fraud was present here."

On Tuesday, the Secretary of State's spokeswoman said the fraudulent petition signature would not impact Keyser's placement on the ballot.

"We have a court ruling that says we must put Jon Keyser on the ballot. Jon Keyser is on the ballot," Bartels said.

Two other Republican U.S. Senate candidates, Robert Blaha and Ryan Frazier, were in Denver court Tuesday, challenging the Secretary of State's decision that they did not collect enough voter signatures to place their names on the ballot for the June 28 primary.

All the Republican candidates are hoping for a chance to challenge Democratic U.S. Sen. Michael Bennet in the November general election.