Trump’s defiance of oversight presents new challenge to Congress’s ability to rein in the executive branch

For the first time since Democrats took control of the House last year, President Trump’s effort to stonewall congressional efforts at oversight have begun to show some cracks.

On Thursday, a former State Department official set off a firestorm when he defied the White House’s no-cooperation strategy and provided Congress with text messages detailing the administration’s effort to leverage a meeting with Trump to pressure his Ukrainian counterpart to launch investigations into the U.S. president’s political rivals.

And House Democrats are expected to interview other critical witnesses this week as...