Republicans need to win only 4 of the 16 competitive Senate races in order to keep control of the Senate, but if you're a conservative, particularly if you're a pro-lifer, you need a lot more than 50 GOP senators.

Here's why: Trump may not be able to appoint a pro-life judge to the Supreme Court with a slim majority, thanks to pro-choice GOP senators such as Lisa Murkowski, R-Alaska, and Susan Collins, R-Maine. Sure, Collins voted for Brett Kavanaugh's confirmation, but Kavanaugh was hand-picked by Justice Anthony Kennedy, who helped save most of Roe v. Wade.

If Trump tried to nominate the likes of federal Judge Amy Coney Barrett, he might not be able to get either of those two.

[Read: The 10 key races to watch: How to know if you're witnessing a blue wave]

So pro-lifers need to get the GOP to at least 52 seats so that a nominee can get 50 without Murkowski or Collins. If Bob Hugin of New Jersey, who is not pro-life, is one of those 52, there still may be trouble. The Republican nominees in the other 16 competitive races are all pro-lifers.

So 53 GOP seats is probably enough to confirm any qualified nominee. That's important because it could open the door for conservative stalwart Justice Clarence Thomas to step aside in favor of a much younger conservative nominee. Barrett is 24 years younger than Thomas, and could assure a conservative majority on the court for longer.

Alternatively, it could allow for Trump to expand the conservative majority — and thus make overturning Roe more likely — if one of the liberal justices were to pass away or had to step down in the next two years.