President Donald Trump Win McNamee | Getty Images

President Donald Trump may not want to make November's midterm elections a referendum on tariffs. Trump is deciding whether to slap duties on another $200 billion in Chinese goods after a public comment period ended at midnight Thursday. He raised the stakes again Friday, saying he has tariffs "ready to go" on an additional $267 billion in Chinese products.

The actions would expand mounting trade conflicts with major partners that have prompted backlash from Republican lawmakers and sparked fears about slower growth and damage to American farmers. China, the European Union, Mexico and Canada have retaliated against Trump's previous trade barriers. Beijing already said it would respond in kind to any new U.S. tariffs. A series of NBC News/Marist polls of six politically important states released in recent weeks highlight the potential political peril Trump faces if the trade war with China affects a larger swath of the economy. Pluralities of registered voters in Illinois, Indiana, Missouri, Pennsylvania, Tennessee and Texas believe tariffs will damage the U.S. economy.

The numbers listed below show the proportion who believe tariffs will "protect American jobs and help the U.S. economy," followed by those who think they will "raise the costs of consumer goods and hurt the U.S. economy," then those who say the actions will not have much of an effect. Illinois: 24 percent/43 percent/20 percent (-19)

Indiana: 29 percent/41 percent/19 percent (-12)

Missouri: 28 percent/44 percent/14 percent (-16)

Pennsylvania: 28 percent/46 percent/14 percent (-18)

Tennessee: 28 percent/41 percent/17 percent (-13)

Texas: 33 percent/40 percent/15 percent (-7) A spreading trade conflict could have ripple effects as Republicans try to defend their House and Senate majorities. Pennsylvania and Texas both hold important Senate races as well as a handful of battleground House contests with mostly Republicans defending seats. Voters in Indiana, Missouri and Tennessee will decide on three of this year's most competitive Senate races, in which Democrats try to hold off GOP challengers. Meanwhile, a few Republican incumbents will also try to keep their seats in Illinois, a major farm state. While NBC and Marist polled those six states in recent weeks, tariffs also could affect important races in farm states such as North Dakota and Montana.