WASHINGTON — 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, Benghazistill 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.