The House Ethics Committee voted unanimously on Thursday to expand its probe of Rep. Charlie Rangel, D.-N.Y., chairman of the powerful Ways and Means Committee who is at the center of an investigation into a long and stinging list of alleged wrongdoing. 

The committee said Thursday that it has reviewed more than 12,000 pages in documents, conducted 34 interviews and held 30 hearings into Rangel's alleged misdeeds, which include failure to pay taxes and disclose income -- as much as $1.3 million -- he earned from multiple properties.  But the committee did not specify how the investigation would change.   

A day after Democrats shot down a measure to remove Rangel from his powerful post as chairman of tax-writing committee, the Congressional Black Caucus sent a letter to  Speaker Nancy Pelosi condemning what it called "partisan attempts to ignore the well-established, bipartisan congressional ethics process."

Republicans in the House failed, by a 243-156 vote, to pass a resolution on Wednesday to oust Rangel from his chairmanship pending an Ethics Committee investigation into alleged tax evasion. 

On Thursday, the CBC wrote a letter to Pelosi blasting the Republican-backed resolution as politically motivated, and said Rangel, a founding member of the CBC who represents New York's Harlem district, has been "subjected to repeated attacks based on allegations that he committed errors in complex financial disclosure and tax filings."

"Regrettably, the minority has repeatedly attempted to make an end-run around the bipartisan procedures for investigating possible ethics issues," the letter read. "These Republican attempts to presume guilt before an investigation has been completed violate the core American principle of the presumption of innocence."

The CBC noted that it was Rangel himself who asked the Committee on Standards of Official Conduct to launch a full investigation into "any possible errors."

"He has demonstrated his cooperation with such an investigation by hiring on his own initiative an accountant to thoroughly review all his records and file corrections when necessary," the CBC said.

Pelosi has said repeatedly that "she wants the Ethics committee to complete the process," her spokesman, Brendan Daly, told FOX News. He declined to comment specifically on the CBC's letter. When questioned by reporters Wednesday about Rangel continuing to serve as chairman, Pelosi said, "Yes, I support him."

Rangel's office lambasted the resolution to oust him as a "highly partisan effort."

"Let's look at this resolution for what it really is -- a highly partisan effort designed to undermine the important work in Congress on health care reform," a Rangel representative, who declined to be named, told FOX News.

"It's also an attempt to circumvent House rules, which provide for a comprehensive, bipartisan ethics committee process for reviewing matters such as these," the aide said. "The congressman himself initiated the request for the committee to review the issues and the members should let the process work as established by the rules of the House."

In a July 2000 interview with FOX News, Rangel dismissed the allegations, saying he is as "clean as the driven snow." 

But Republicans, including House Minority Leader John Boehner, R-Ohio, have steadfastly called for Rangel to step down pending the results of the investigation. The measure's defeat is "the latest example of Speaker Pelosi breaking her promise to have the most 'open and ethical' Congress in history," Boehner said in a statement.

"Instead of holding Chairman Rangel accountable for his actions, House Democrats are once again circling the wagons and demonstrating their loyalty to a leader who faces serious questions about his official conduct," Boehner said.

Wednesday's vote came as reports emerged that Rangel funneled a $3 million earmark -- included in the massive Defense Appropriations Bill -- to City College of New York to fund research on materials used to protect Army vehicles from attack.

Critics charge that the powerful chairman of the tax-writing committee -- arguably the most influential committee in Congress -- should step down amid growing allegations and investigations into possible ethics violations.

Rangel, who has served in the House for nearly 40 years, reportedly failed to report as much as $1.3 million in income from 2002 to 2006. In 2008, the New York Post reported that he failed to disclose $75,000 in rental income.

In June 2009, the House Ethics Subcommittee opened a probe of Rangel's trips to conferences in the Caribbean, and in August Rangel disclosed more than $500,000 in previously unreported assets.

"These are glaring tax violations and he has paid no penalties or interest," said John Stone, a spokesman for Rep. John Carter, R-Texas, who is leading the effort to oust Rangel from his post.

"It's setting up a double standard," said Stone. "The average American gets slapped with fines and possible jail time, while the rich and powerful can get off 'scot-free.'

"He's chair of the very committee that oversees internal revenue of the tax code. He simply cannot serve with this massive question and these constantly expanding investigations," Stone continued.

Of the $3 million earmark directed to City College, he said, "This is a prime example of why he should step down as chair until these ethics questions are answered."

On Wednesday, House Majority Leader Steny Hoyer, D-Md., said, "The [Ethics] Committee is doing its work. We will wait for its report."

"To do anything else would be premature. I, or someone, will move to table the resolution," Hoyer told reporters.

FOX News' Chad Pergram contributed to this report.