Former U.S. Ambassador to Mexico Roberta Jacobson says that a victory by leftist presidential candidate Andres Manuel Lopez Obrador -- known as AMLO -- in Mexico's general election could lead to a more positive relationship with the United States, even as President Trump and Mexican officials continue to butt heads over regional issues like trade, drug trafficking and the border wall.

"One of the things that he [Lopez Obrador], both in my discussions with him and many of his conversations and advisers conversations ahead of these elections have emphasized is the relationship with the United States and that it be positive," Jacobson said on "Face the Nation" Sunday. "They're going to work hard on that, which does not mean it's going to be easier than it has been with current Mexican government. I think there a number of issues on which it's going to be difficult and maybe harder."

She added that Lopez Obrador "has been at pains to reassure people that he takes this relationship seriously, that he does not think that it needs to be -- descend into insult, and he has the leftist credentials to stand up politely in a way that I think he's got the credibility to do that."

What's at stake in Sunday's elections is whether or not the U.S. continues to cooperate and work with Mexico as a partner, according to Jacobson, including areas like the economy, immigration and security. She said the most important aspect to consider is "whether that partnership we built over the last 30 years begins to deteriorate."

Jacobson said Mr. Trump's harsh rhetoric toward Mexico "is not helping make this partnership any better." His rhetoric was a major factor in Jacobson's decision to leave the administration after serving in the foreign service for 31 years.

"Being ambassador to Mexico was my dream job," Jacobson said. "I loved the work. I still love the work, but it became increasingly difficult to do under this administration, because every time you tried to do something on NAFTA or on security or on any of the most important issues to us, education, et cetera, things would get blown up by a tweet."

She added, "there were many people in government trying to do the right thing because this partnership is important to Americans and yet it just never seemed that we could overcome the retreat into the vilification of Mexicans at a rally or a tweet and that was really difficult."