Someone with many great ideas/arguments, but who ruins their case with one very bad idea/argument [closed]

Solution 1:

Someone may undermine his/her own credibility.

Trump's recommendation against dental floss undermines his well-thought out positions on dental care.

Solution 2:

You might say this this person has gone a bridge too far.

From wikipedia:

An idiom inspired by Operation Market Garden, meaning an act of overreaching

This event also inspired a book:

A Bridge Too Far, a non-fiction book by Cornelius Ryan published in 1974, tells the story of Operation Market Garden, a failed Allied attempt to break through German lines at Arnhem across the river Rhine in the occupied Netherlands during World War II in September 1944. The title of the book comes from a comment made by British Lt. Gen. Frederick Browning, deputy commander of the First Allied Airborne Army, who told Field Marshal Bernard Montgomery before the operation, "I think we may be going a bridge too far."

And the book inspired a movie:

The name for the film comes from an unconfirmed comment attributed to British Lieutenant-General Frederick Browning, deputy commander of the First Allied Airborne Army, who told Field Marshal Bernard Montgomery, the operation's architect, before the operation: "I think we may be going a bridge too far."

Edit: You also might consider:

One rotten apple spoils the whole barrel:

A single bad influence can ruin what would otherwise remain good.

This can also be stated as:

• One rotten apple spoils the bunch.
• One rotten apple ruins the rest.

"He has a lot of good ideas, but his idea about free ponies is a rotten apple that spoils the rest."

Solution 3:

Another foot-related phrase: Achilles' heel

"Gun-control is Sanders' Achilles heel."

m-w.com defines it as such: "a fault or weakness that causes or could cause someone or something to fail".

I like to think it also implies strength or the perception of strength in all other areas because of the original myth.

Solution 4:

Consider, a fly in the ointment

A spoiler. Ecclesiastes 10:1 relates that dead flies impart a bad odor to perfume; early versions translate the word “perfume” as “ointment.” Another old phrase with the same meaning is to “throw a monkey wrench in the works.” (Endangered Phrases by Weston A. Price)

Bernie Sanders' only fly in the ointment is gun control.