CNBC's Jim Cramer on Friday drew a line in the sand on President Donald Trump's attempt to negotiate trade via Twitter.

After supporting a hard-line trade stance against China, Cramer denounced the surprise 5% tariffs that Trump has threatened to slap on all imports from Mexico starting next month. The threat sent the three major U.S. indexes down between more than 1% each Friday.

"When Trump goes after the Mexican government, which has already made a bunch of concessions in the NAFTA renegotiations, it makes investors nervous [and] makes them want to sell," the "Mad Money" host said. "It feels like there's no plan, like anything could happen, and that's a bad feeling because the markets hate uncertainty."

America imported more than $346 billion of merchandise from its southern neighbor in 2018, according to the Office of the U.S. Trade Representative. Mexico is the country's second largest trade partner, and the duties on imports could reportedly cost U.S. consumers at least $18.6 billion.

The Trump administration said the tariffs would put pressure on the Mexican government to impede the flow of undocumented asylum seekers crossing the border. Trump said he has planned a series of tax hikes over the next several months if the matter is not addressed to his satisfaction.

"If you want a strong economy and higher stock prices ... the last thing you need is a president who bursts through Twitter shouting: 'Who's next for punishment?' ... You can't conduct negotiations out of left field, via tweet," Cramer said. "In an era where all that matters is who's next for punishment ... we need to be very cautious, unless we get some sort of game-changing data that shifts the whole narrative."

Here's what Cramer is watching in the week ahead: