WASHINGTON — Republicans’ expansion of their Senate majority means that the conservative legal movement is poised to expand and entrench its influence over the federal courts, leaving Democrats with dwindling hopes of being able to swiftly diminish that imprint even if they win the White House in 2020.

The net pickup by Republicans of about three seats leaves President Trump and the Senate majority leader, Mitch McConnell, positioned to accelerate their already record-breaking pace of appointing federal judges through at least 2020. The White House and Mr. McConnell will now have more running room to get young and outspoken conservatives through even if a few Republicans break ranks, and they can consolidate the new conservative majority on the Supreme Court if a justice leaves during the next two years.

But the midterm results have longer-term implications as well. By broadening their Senate majority from 51 seats to about 54 (some races have yet to be decided), Republicans have expanded their chances of retaining control of the chamber after 2020. Only a handful of Republican Senate incumbents will be up in states where Democrats are competitive, and they have their own vulnerable incumbents, too.

As a result, even if a Democrat defeats Mr. Trump and takes over the White House in 2021, Mr. McConnell is likely to retain sufficient power in the Senate to prevent that president from appointing judges to start swinging the pendulum back. That situation would be similar to what happened in President Barack Obama’s final two years in office, when Republicans, after taking control of the Senate, systematically blocked his nominees to fill vacancies on both federal appeals courts and the Supreme Court.