House Republicans are set to begin a special investigation of the deadly attack on the U.S. diplomatic post in Benghazi, Libya, raising the stakes in a political battle with the Obama administration as the midterm election season heats up.

Democrats are considering a boycott of the committee, which is expected to be approved formally when the Republican-led House votes Thursday afternoon. They don’t want their presence to provide legitimacy to what they believe will be a partisan forum, yet they don’t want to lose the ability to counter Republican claims and provide cover for potential witnesses.

Speaker John Boehner vowed Wednesday that the examination would be “all about getting to the truth” of the Obama administration’s response to the attack and would not be a partisan, election-year circus. “This is a serious investigation,” he said while accusing Obama and his team of withholding the true story of how militants killed Ambassador Chris Stevens and three other Americans on Sept. 11, 2012.

Democrats voiced wide-ranging concerns over the scope and composition of the select committee. They said they’d make no decision on whether to participate in the panel until Boehner responds to a demand from Minority Leader Nancy Pelosi that he scrap his plan for a committee of seven Republicans and five Democrats. Democrats say membership should be evenly split, and want clearer time and cost constraints for a forum they likened to a “kangaroo court.” Rep. Xavier Becerra, D-Calif., said Boehner’s panel “can go on forever.”

Republicans have made Benghazi a central plank of their strategy to wrest control of the Senate from the Democrats in November’s traditionally low-turnout midterm elections. Twenty months since the attack, Benghazi still revs up the GOP’s conservative base.

Republicans say the White House, concerned primarily with protecting President Barack Obama in the final weeks of his re-election campaign, misled the nation by playing down intelligence suggesting Benghazi was a major, al-Qaida-linked terrorist attack. They accuse the administration of stonewalling congressional investigators ever since, pointing specifically to emails written by U.S. officials in the days after the attack but only released last week.

“A line was crossed,” said Boehner, who in April declared he saw no need for a select committee. Correspondence among top officials showed the White House “played a more significant role” in deciding how the attack ought to be described publicly, he told reporters Wednesday.

The Obama administration says officials tried to provide the public with the best information available after the attack at a time when U.S. embassies, consulates and other facilities were facing angry demonstrations across the Muslim world over a YouTube video mocking Islam’s Prophet Muhammad. It originally attributed Benghazi to a similar protest that extremists hijacked, but retracted that account amid severe criticism. It says Republicans are persisting with Benghazi questions in the hopes of generating a scandal to gain political support.

Like House Democrats, the administration has yet to say if it will cooperate with the select committee. On Wednesday, White House spokesman Josh Earnest pounced on congressional Republicans for seeking campaign money off the investigation, describing a fundraising email earlier in the day as a “pretty good indication of political motivation.”

The National Republican Congressional Committee’s pitch said the GOP was “moving fast” to hold Obama and former Secretary of State Hillary Rodham Clinton “accountable for their actions” on the night of the Benghazi attack. It vowed that “no one will get away” from the select committee and asked people to become a “Benghazi Watchdog” by donating money. Suggested contributions started at $25.

Asked about fundraising in an interview, Rep. Trey Gowdy, the South Carolina Republican chosen by Boehner to head the investigation, said using Benghazi was a bad idea. “I have never sought to raise a single penny on the backs of four murdered Americans,” he said Wednesday on MSNBC’s “Morning Joe.”

(AP)