Trump urges voters to vote for Senator Strange in Alabama Senate primary

United States President Donald Trump has tweeted his support for Senator Luther Strange as polls opened on Tuesday morning for the hotly contested Alabama Republican runoff for the US Senate.

Trump endorsed Strange over Roy Moore to fill the seat which Attorney General Jeff Sessions had held for two decades.

"Big election tomorrow in the Great State of Alabama. Vote for Senator Luther Strange, tough on crime & border - will never let you down,"Trump tweeted.

Strange, who was appointed by then-Governor Robert Bentley to finish out the term after Jeff Sessions became attorney general, is running against Moore, the former chief justice of the Alabama Supreme Court in the GOP primary.

"They think you're a pack of morons. They think you're nothing but rubes," CNN quoted Steve Bannon, Trump's campaign chief executive and until recently the chief strategist of the White House.

Whoever wins the primary will advance to the December 12 special election to face Jones who has the backing of former Vice President Joe Biden.

A primary win by Moore would open Alabama's US Senate seat to Democrats, Trump said.

"Luther Strange is going to be a great senator. He loves Alabama, he loves the states and he loves the country. He will absolutely win against the Democrat. Ray (sic) will have a hard time. If Luther wins, the Democrats will hardly fight. If Ray wins (Democrats) will pour in $30 million,"CNN quoted Trump as saying.

Opposing Republican candidate endorsed by Donald Trump, the former White House adviser Steve Bannon insisted, "We did not come here to defy Donald Trump - we came here to praise and honour him.