A photog caught up with the South Carolina congresswoman as she made her way to Capitol Hill on Monday, where she shared her candid opinion on Biden dropping out of the 2024 Presidential race ... and why it's more serious than people realize.

As NM put it ... we should be invoking the 25th amendment, which could see President Biden's powers transferred to Vice President Kamala Harris if he's declared unfit to serve.

She explained ... "If you're not competent enough to run for office, you're not competent enough to run the country. You can't call for his dropping out of the presidential election and be consistent and say he should be running the country. That's just complete hypocrisy."

The congresswoman meant business too -- watch ... she says she's filing a resolution later today on this matter.

Of course, NM first had to make a stop at Secret Service Director Kimberly Cheatle's congressional testimony. The congresswoman came out swinging during the hearing ... grilling KC for her lack of insight into the assassination attempt on Donald Trump.

In fact, the U.S. rep called out KC for leaking her opening statements to the press hours before sharing them with the committee -- and when Cheatle claimed ignorance on how it leaked, Mace quite literally called BS.