Bartlett, R-Md., said "a flurry" of news stories about Otis' congressional aspirations made it impossible for Otis to continue working for Bartlett.
Otis submitted his resignation Wednesday night, effective Thursday. Bartlett said he didn't ask for Otis to resign.
By then, Republicans David R. Brinkley and Alex X. Mooney already had taken steps toward running for the 6th District seat.
Mooney, a former state senator representing Frederick and Washington counties, has long said he only would pursue the seat if Bartlett didn't run again.
Brinkley, a state senator representing Carroll and Frederick counties, hasn't made his interest in the seat contingent on Bartlett's plans.
On Thursday, Mooney and Brinkley both released statements that they were forming exploratory committees, allowing them to raise money before becoming official candidates for Congress.
Brinkley released a list of Republicans who he said back his campaign, including three in the Washington County delegation — Sen. Christopher B. Shank, Sen. George C. Edwards and Del. LeRoy E. Myers Jr. Shank and Myers previously have said they were considering running for Congress, but later decided against it.
Otis acknowledged Thursday that he, too, has thought about running for the seat, depending on what Bartlett decides.
Otis said he has worked for Bartlett for 11 years and was his chief of staff for about nine or 10 years.
He said that during the past year, he thought it might be time to move on to something else; the frenzy over his possible candidacy accelerated his departure.
Otis said the conversations he had in the last few weeks about possibly running for Congress were prompted by people asking him to consider the seat if Bartlett didn't run. Each time, he said, he told inquirers that Bartlett planned to run.
Asked if Bartlett was aware of those conversations, Otis said, "He knew this."
Bartlett said he didn't know about Otis' candidacy conversations at first, but "by and by, it was kind of common knowledge."
"I regret that misunderstandings in the press created this kind of environment," Bartlett said.
Asked what he meant by that, Bartlett said Otis' conversations were based on a hypothetical question, but were exaggerated into "something more imminent."
Otis said the possibility of him running for the 6th District seat "hinges on further clarification" by Bartlett about his plans.
Although Bartlett, 85, has said numerous times that he's running for an 11th term in 2012, some Republicans have been skeptical, based on his lackluster campaign activity, particularly fundraising.
Last month, addressing the possibility that he might back out of the 2012 race, possibly shortly before the filing deadline, Bartlett said in an interview, "At some point, I will decide that I'm not running. I have no idea when that point will come. As I said, it depends on a lot of things, and one of those is my health and my family."
On Thursday, Bartlett reiterated his plans to run again next year. He said the fact that he just raised $150,000 to support a lawsuit challenging the state's new congressional boundaries shows he's capable of raising money when he needs to.
Still, speculation remains that Bartlett isn't engaged in what is shaping up to be a rugged campaign season. The recent congressional redistricting in Maryland, assuming it withstands court challenges, adds more of Montgomery County into the 6th District, making it easier for a Democrat to succeed.
Robert J. Garagiola, the state Senate majority leader, is seen as a top Democratic candidate and has been actively campaigning.
Democrats Duchy Trachtenberg and Dr. Milad L. Pooran also are in the race, along with Republican challengers Joseph T. Krysztoforski, Robin Ficker, Robert Coblentz and Brandon Orman Rippeon.
Mooney, who filed paperwork for his exploratory committee on Wednesday and announced his decision on Thursday, said, "You have to define the term 'running.'"
There's a difference, he said, between filing paperwork and campaigning robustly; he hasn't seen any signs that Bartlett is committed to another race.
Hearing that Otis was gathering support for a possible candidacy, he decided to do the same.
Mooney said he plans to raise a lot of money in December, then decide shortly before the Jan. 11, 2012, filing deadline if he wants to become an official candidate.
If he does, he'd step down as the Maryland Republican Party chairman and take a leave of absence from his job as executive director of the National Journalism Center.
Brinkley couldn't be reached for comment on Thursday, but Don Murphy, a former delegate serving as a campaign spokesman, said Brinkley was planning to run for the 6th District seat anyway.
But when the news about Otis broke, Brinkley took action and rounded up local supporters.
"It is probably what caused us to pull the trigger when we did," Murphy said.
Murphy said Brinkley expects to file papers for his campaign committee as soon as Friday.
With the new congressional boundaries, Brinkley no longer lives within the 6th District, but members of Congress aren't required to live in the districts they represent.
Mooney wondered why Brinkley wouldn't run in the 8th District, where he lives.
Murphy said that isn't necessary.
"He didn't leave the 6th District," Murphy said of Brinkley. "The 6th District left him."