In the run-up to the Iraq war, the Democrats were doing exactly what you think

Which was ignoring the fact that they knew the administration was lying. Dick Durban, in the link, tells the story of how as a member of the Senate Intelligence Oversight Committee, he was not led to believe there were nuclear, biological or chemical weapons in Iraq. He saw the American people being so led, and made no counter claim.

Congress' one out on the culpability for this failed venture has been that they were lied to just as we were, and voted to overturn the War Powers Act because they believed the danger from Saddam Hussein was so urgent that there would never be time to consult them before invading.

Durbin's just given that up, unless oversight really means sitting on your hands while watching villainy. But, then, one might ask, why have it?