Story highlights President Donald Trump recorded a robo-call for Sen. Luther Strange

He faces a three-way primary Tuesday against Roy Moore and Mo Brooks

Birmingham, Alabama (CNN) President Donald Trump and Senate Republicans are making a major last-minute push to save Sen. Luther Strange in Tuesday's three-way primary here.

Strange is trying to finish in the top two -- and make sure none of his other challengers top 50% of the vote -- to advance to a primary runoff in the race to fill out the rest of Attorney General Jeff Sessions' Senate term.

But GOP strategists involved in the race tell CNN that they believe the controversial former state Supreme Court Chief Justice Roy Moore is likely to secure a spot in th runoff and could approach 50% of the vote. Rep. Mo Brooks, meanwhile, is attempting to damage Strange by exploiting Strange's two politically problematic links: to Senate Majority Leader Mitch McConnell and to former Gov. Robert Bentley, who appointed Strange, then the state's attorney general, to fill Sessions' seat until the special election just before Bentley was ousted amid a sex scandal.

Trump tweeted an endorsement of Strange for the second time Monday morning.

"Luther Strange of the Great State of Alabama has my endorsement. He is strong on Border & Wall, the military, tax cuts & law enforcement," the President said.

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