An Inexperienced President

The "most unqualified Presidential Candidate ever nominated by a major political party throughout USA's modern political history" was a Republican.

Before he even became a Republican, he served a single 2-year term in the House of Representatives. He was gaffe prone and was such a political failure that he didn't even run for reelection.

After that, he worked as a lawyer for a decade and made a few speeches, but generally stayed away from politics entirely. After he became a Republican, he lost a bid for the Senate to an ex-Democrat.

After only two years experience and 11 years removed from any elected office, he decided to run for president. He was by far the underdog, but the Democratic party at the time was so completely fractured. He won, but generated less than 40% of the popular vote.

His name was Abraham Lincoln.