The US goes to the polls in the first major electoral test for Trump since he took the White House and embarked on an "America First" agenda that has split opinion across the country and globally.

While his tax cuts and deregulation have helped fire the economy and push stock markets to multiple record highs earlier this year, there is a growing concern that his long-running trade row with China is beginning to bite.

The vote has the added twist of an investigation that is looking at whether his campaign colluded with Moscow to win the 2016 election. If the Democrats win both houses of congress, they could push harder for impeachment, fuelling uncertainty.

"These midterm elections carry a sizeable legal risk for the (Republicans) which could dent investor confidence as we will likely hear much more from (Russia probe lead counsel) Robert Mueller sooner rather than later," said Stephen Innes, head of Asia-Pacific trade at OANDA.

In early trade Hong Kong was down 0.2 percent, having lost more than two percent on Monday, while Shanghai edged down 0.6 percent and Taipei lost 0.5 percent.

However, Tokyo was up more than one percent by the break as a weak yen supported exporters, Sydney gained 0.8 percent and Seoul added 0.4 percent. Wellington rose 0.5 percent, Manila put on 0.6 percent and Jakarta climbed 0.3 percent.