WASHINGTON — President Trump and Republicans in Congress say they are committed to protecting people with pre-existing medical conditions. But patients with cancer, diabetes and H.I.V., for example, would have significantly less protection under Republican proposals than under the Affordable Care Act.

The proposals may provide some political cover for Republicans on an issue likely to figure prominently in the 2020 elections. But a close inspection of the Republican bills shows that their protections are undercut by a combination of imprecise language, explicit exceptions and “rules of construction” that explain how the legislation is to be interpreted.

“We will always be protecting pre-existing conditions,” Mr. Trump said this past week in a visit to Minnesota, echoing a vow he has made at least 20 times in the past six months. “Remember that. That’s No. 1. Very, very important.”

A recent bill, introduced by Senator Thom Tillis, Republican of North Carolina, and endorsed by 22 other Republican senators, would prohibit insurance companies from denying coverage because of a person’s pre-existing conditions.