WASHINGTON -- Republicans in Washington and Miami are growing increasingly anxious about the ongoing state investigation into Rep. David Rivera, with some urging him to explain his finances while others are already talking of potential GOP successors to replace him.

House Republicans have said privately they're frustrated with the drumbeat of media attention surrounding the freshman lawmaker. This week, newly elected House Speaker John Boehner was confronted with a question about Rivera at a Capitol Hill news conference, while a prominent Republican attorney in Miami called for Rivera to fully explain himself or "step down.''

In a letter to the Miami Herald, Thomas Spencer, who is active in Miami-Dade and state Republican politics, said Rivera needs to ``fully and completely, without delay or obfuscation, disgorge and fully explain every single relevant fact and document -- or he needs to step down.

"Litigation will only erode the reputation of our community in the United States House of Representatives and impugn the good name of our Congressional delegation -- all of whom have brought pride for their service to their districts,'' said Spencer, who was a co-counsel for former President George W. Bush during the 2000 Florida recount.

Spencer, who said he's talked to a number of other Republicans, said he plans to call on top Republicans to ask Rivera to release more details about the investigation. ``It's time for us to step up to the bat and put pressure on him to get him to fork over every single document,'' he said Friday.

Rivera's congressional campaign said in a statement that he's ``reassured'' colleagues and congressional leaders and is "confident he has not committed any wrongdoing whatsoever.''

The campaign included a copy of Rivera's 2010 federal financial disclosure, which had been submitted to the House days before his swearing in, calling it ``a testament to his pursuit of complete financial transparency and accountability.

The statement said Rivera will "continue to focus his energies entirely on the issues that affect the residents of Florida's District 25, job creation and economic growth.

"He will certainly not be distracted by attacks which the public understands are politically-motivated and based on unfounded allegations and speculation,'' the campaign said.

Rivera's office says he was recruited to be a leader of the House's Free Trade Caucus and is working with House leadership on Colombia, Panama and South Korea trade agreements. And he and Florida Sen. Marco Rubio are scheduled next month to host the county GOP's Lincoln Day dinner.

Prosecutors and detectives from several agencies -- the Miami-Dade State Attorney's Office, the Miami-Dade Police Department's public corruption squad and the Florida Department of Law Enforcement -- began investigating Rivera's finances before he was elected to Congress in November.

At the heart of the probe is Millennium Marketing, a company owned by Rivera's mother and godmother that received $510,000 from the Flagler Dog Track as part of a deal for Rivera to lead a pro-slots political campaign on behalf of the parimutuel.

Rivera, who had long denied receiving any money from the dog track, earlier this month admitted to receiving $132,000 in undisclosed loans from Millennium -- loans Rivera says he has since repaid.

Also under investigators' microscope: Rivera's campaign expenses, including $30,000 he paid to Millennium for consulting in 2006, and $75,000 he paid last year to a now-defunct consulting company owned by the daughter of a longtime aide. Rivera has denied any wrongdoing.