Scrutiny of the president increases with a divided government (different parties in Congress and presidency).
A divided government makes it harder for the president to pass legislation.
Opposition control of the Senate makes it harder for the president to get nominations confirmed or treaties ratified.
Examples of Congress control
President Obama could not have his judicial nomination, Merrick Garland, confirmed to the Supreme Court because the Republicans had control of the Senate.
The Republicans refused to hold a hearing or vote on the nomination.