House Minority Leader Nancy Pelosi (D-Calif.).  (Shawn Thew/ European Pressphoto Agency)

Updated at 10 a.m.

Top House Democrats sent a letter to Speaker John A. Boehner on Tuesday night, protesting the proposed structure of the House select committee being set-up to investigate the 2012 attacks in Benghazi, Libya, the latest in a series of moves that seem to signal that Democratic participation in the committee is unlikely.

The letter, sent by House Minority Leader Nancy Pelosi (D-Calif.) and Minority Whip Steny Hoyer (D-Md.), decries the proposed structure of the committee, which would be 12-member paneled comprised of seven Republicans and five Democrats.

Pelosi and other top Democrats had called for an even split between Democrats and Republicans, declaring that would be the only way that the committee could be "fair."

"If you truly want this new select committee to be bipartisan and fair  — and to be taken seriously by the American people  — we call on you to reconsider this approach before bringing this measure to the House floor for a vote," the letter sent to Boehner says. "Another partisan review that serves only to politicize these attacks is disrespectful and unworthy of the American people."

The House is expected this week to vote on  — and approve  — the proposed select committee, which would further examine the Obama administration's response to the 2012 attacks that left four Americans dead.

Republicans allege that the Obama administration deliberately coordinated inaccurate talking points about the cause of the attacks, which came just weeks prior to the 2012 presidential election, in order to insulate the president from political criticism.

Some House Republicans been calling for a select committee investigation for months, but Boehner (R-Ohio) had resisted. However, that changed last week when GOP furor over Benghazi was renewed upon the release of new e-mails that some of the right allege are a smoking gun, proving the White House knowingly spread misinformation about the attacks.

Boehner released the text of the resolution chartering the committee Tuesday night, showing that the split would be seven Republicans and five Democrats, prompting several top Democrats to call for their party to boycott the committee.

"I would be dead-set against it. We should not even participate in it," declared Rep. James Clyburn, (D-S.C.), a member of the Democratic leadership, as he spoke to reporters  Tuesday night. "Let them drive it, they're driving it anyway … I'm not bringing a noose to my hanging."

Other Democrats have questioned what he scope of the proposed committee, saying that they have yet to hear a full articulation from the GOP leadership of why this new committee is necessary.

"What else are we looking for? What is the mission of this new committee?" said Rep. Dutch Ruppersberger (D-Md.), the ranking Democrat on the House Intelligence Committee, while speaking to reporters Tuesday night. "What else do we need to find. The American people need to comfortable that we've done every thing we can, and the most important issue is making sure that we get the bad guys who did this."

Boehner on Wednesday rejected the Democrats' criticism, saying the special committee would consolidate the work of four other panels on the attacks.

He suggested that recent revelations of White House e-mails required the House to keep probing but in a unified fashion. "It's clear that we need to proceed. … A line was crossed last week," Boehner told reporters after his weekly closed-door meeting with the GOP caucus.

While the Democrats have yet to officially say they will not participate in the committee, the 7-to-5 split seems to indicate that a bipartisan panel is increasingly unlikely.

Even prior to the announcement of the membership structure, Democratic aides were floating the possibility that Democratic members of Congress would protest the panel.

On Monday, a House leadership aide told The Post's Greg Sargent that Democrats were considering a protest, pointing to their decision to not participate in a 2005 GOP-run committee on how the Bush administration handled Hurricane Katrina.

At the time, Democrats argued that the House GOP had set up the committee in a way that ensured it would not conduct a serious probe.

“There is deep concern in the Caucus that participation in this sham committee, like the 2005 Katrina committee, would serve to legitimize what has and by all signs will continue to be a political operation,” the Dem leadership aide told Sargent.

Republican aides, meanwhile, have consistently noted that a 2007 House select committee, commissioned by Pelosi, when she was House speaker, was structured with nine Democrats and six Republicans.

In an interview on MSNBC on Wednesday morning, Rep. Trey Gowdy (R-S.C.), who Boehner has announced will lead the Benghazi committee, dismissed the Democrats' calls for an even split.

"Elections have consequences," Gowdy said. "Just because a committee is evenly distributed doesn't necessarily enhance its credibility. It can be fair without being even."

Paul Kane contributed to this report.