Who was the last u. S. President to run effectively unopposed?

President James Monroe in 1820

Explanation:

The Democratic-Republican President John Monroe was reelected into office unopposed. His opposition could not bring a credible candidate to challenge the incumbent (already sitting president) and as a result, the election was unopposed and he won the necessary Electoral Colleges and was returned President.

The inability of the opposition, the Federalist Party to find a suitable and credible candidate to oppose President John Monroe was as a result of its dwindling influence in the political landscape after the war of 1812.

The incumbent President won all the available states with an 80% margin. His running mate and vice president was Daniel Tompkins.