President Trump's approval ticked up to 49 percent - its highest mark this year, according to a new Hill-HarrisX survey released on Wednesday.

The figure marks a 2-point increase from a Sept. 11-12 poll, but a 2-point decrease from its previous peak of 51 percent in August 2018.

Trump's disapproval rating, meanwhile, dropped to 51 percent, which marks his lowest level so far this year.

The nationwide survey was conducted on Sept. 28 and 29, less than a week after House Democrats launched a formal impeachment inquiry into Trump over concerns raised in a whistleblower's complaint about the president's communications with Ukraine.

House Democrats threatened Wednesday morning to subpoena the White House for documents related to Trump's dealings with Ukraine as part of their impeachment inquiry.

House Oversight Committee Chairman Elijah Cummings (D-Md.) said in a memo that House committees have repeatedly tried to obtain voluntary compliance from Trump officials, but the White House has "refused to engage with - or even respond to - the Committees."

Speaker Nancy Pelosi (D-Calif.) and House Intelligence Committee Chairman Adam Schiff (D-Calif.) held a joint news conference later that morning, warning that attempts by the White House to "stonewall" the impeachment inquiry and "conceal facts" would be considered an obstruction of justice.

President Trump, meanwhile, has warned about the implications of a potential impeachment, and claimed that Democrats are just trying to hurt the country.

"The Do Nothing Democrats should be focused on building up our Country, not wasting everyone's time and energy on BULLSHIT," Trump tweeted following the news conference.

HarrisX researchers surveyed 1,000 registered voters. The margin of error for the full sample is plus or minus 3.1 percentage points.

-Tess Bonn