Jim Shepard, a 10-year veteran of Capitol Hill who was the congressman's executive assistant and scheduler from March to the end of June, told the Las Vegas Sun he witnessed Porter making the calls on five different dates.
Such calls would violate federal election laws and House ethics rules.
Porter's top congressional aide, Mike Hesse, called the accusations "completely false and baseless."
"They rely solely on the claims of one former, disgruntled staffer who is lying. I believe that unbiased observers will question the legitimacy of the story and its sole source," he said in an e-mail.
Shepard said he wrote an e-mail to Hesse about the problem on June 18, saying, "We can NOT let him do this anymore."
But the former staffer said Hesse responded by chastising him for putting his concerns in writing.
Hesse said he had no recollection of the message.
Federal law makes it illegal for members of Congress to solicit campaign donations in federal buildings. The law carries a penalty of up to three years in prison and a fine of up to $5,000.
U.S. House of Representatives ethics rules also ban soliciting campaign contributions in or from any House office, room or building.
Shepard said he witnessed Porter make five to 15 fundraising calls with a campaign cell phone from his Henderson office on each of April 17, 18 and 21 in anticipation of President Bush's fundraising visit to Las Vegas on April 24.
On two days in mid-June, Porter, again using a campaign phone, made fundraising calls from his office in Washington, Shepard said.
During some of those calls, Shepard said he took notes while sitting directly across from Porter at his desk. Porter used call sheets provided by his campaign, which included specific amounts of money that Porter would request the potential donor to contribute, Shepard said.
"He made fundraising calls both from his D.C. congressional office and his district office in Henderson," said Shepard. "While I worked for him, he did it at least five times. I was personally there."
Now a consultant living in Virginia, he said he is not working on any Nevada campaigns this fall.
Experts agreed any campaign fundraising calls made from Porter's official government offices would violate the law.
"Anyone who ... even makes fundraising phone calls from a federal office is violating the law," said Paul Herrnson, director of the Center for American Politics and Citizenship at the University of Maryland. "Members of Congress are well aware that it is illegal."
Porter's opponent for the seat in the November elections, Tessa Hafen, said in a statement that the accusations "raise real questions about his character and about whether or not he should be returned to office."