President Donald Trump’s approval ratings are low enough to give Republicans major stress about holding onto control of the House next year — and to make Trump nervous about impeachment.

Gallup’s most recent daily tracking numbers have Trump’s approval at 38%, which is the worst at this juncture of a presidency since they began collecting such data during the Truman administration. In 2010, Gallup’s Jeffrey M. Jones conducted a study on the impact on midterm elections of a presidential approval rating below 50%. He found that from the Truman to George W. Bush administrations, approval under 50% led an average loss of 36 seats in the House of Representatives. President Obama, with approval ratings below that threshold each time, saw Democrats lose 63 seats in 2010, and 13 in 2014.

Democrats need to pick up 24 seats (23 if Georgia Democrat Jon Ossof wins his special election runoff) in order to regain control of the House.

A breakdown of seat losses over time against presidential approval rating.

The GOP, for its part, has been raising money for Congressional races by pointing out the need to defend Trump from impeachment.

A president can be impeached by a majority vote in the House; he then goes to trial in the Senate. Andrew Johnson and Bill Clinton are the only presidents ever impeached. Both were acquitted in the Senate and remained in office.