House Speaker John A. Boehner laid out details of his proposed Benghazi investigative committee Tuesday night, calling for a 7-5 split between Republicans and Democrats — drawing a stern response from Democrats who said that is unfair and taints the investigation from the start.

Mr. Boehner said the committee won’t have a set deadline for finishing its investigation, but will go until it has the answers to questions about the run-up to the 2012 terrorist attack in Libya, the administration’s handling of the attack, and the White House’s effort to shape public perception of the attack afterward.

“It is unfortunate that it has to come to this, but when four Americans are killed by terrorists in a well-coordinated assault, the American people will not tolerate the evasion we have seen from the White House,” the speaker said.

He said the investigation had to have an open-ended timeframe because the Obama administration has demonstrated it isn’t willing to cooperate.

His resolution, which Republicans are preparing for floor action later this week, would give the committee chairman — whom Mr. Boehner has said would be Rep. Trey Gowdy — the power to issue subpoenas.