(Reuters) - At first glance, U.S. Representative Mo Brooks seems exactly the kind of candidate President Donald Trump would love to see win Tuesday's Republican primary election for Alabama's open U.S. Senate seat.

The 63-year-old Republican is a Freedom Caucus member and an immigration hardliner who calls opponent Luther Strange "Lying Luther," echoing Trump's penchant for bestowing insulting nicknames on his political foes. Strange, 64, is the former state attorney general who was appointed to fill the Senate seat left vacant after Jeff Sessions became U.S. attorney general.

"Trump would like to drain the swamp; Brooks would like to blow it up," said Larry Powell, a professor of communication at the University of Alabama at Birmingham. "But their goals are the same."

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Yet it was Strange, not Brooks, who earned a coveted prize last week in a race that could measure Trump's influence in a state he carried easily in last year's election, despite recent indications that his support among Republicans may be softening.

Trump took to Twitter to offer Strange his "complete and total endorsement."

The tweet came as something of a surprise. Known as "Big Luther" thanks to his 6-foot-9-inch (2.1-meter) frame, Strange has a close alliance with Senate Majority Leader Mitch McConnell, who is under fire from Trump for failing to push his agenda through Congress and refusing to eliminate a filibuster that gives Democrats veto power over many key bills. Strange agrees the filibuster should remain in place.

In a telephone interview, Brooks noted that Trump followed his endorsement of Strange with a barrage of Twitter criticisms aimed at McConnell, which Brooks called "extraordinarily baffling."

"Any Alabama voter who wants to see President Trump's legislative agenda pass the United States Senate would be much better served to vote for Mo Brooks than Luther Strange," Brooks said. "President Trump's entire legislative agenda is dead so long as the Senate's 60-percent rule requires President Trump to get the consent of Democrat leader Chuck Schumer to pass it."

Both Brooks and Strange may be losing to the third viable candidate in the nine-way race. Former Alabama Supreme Court Chief Justice Roy Moore, 70, has led in several voter surveys, though polling in the race has been limited.

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The Moore and Strange campaigns did not respond to requests for comment.

Assuming no candidate reaches 50 percent of the vote, the top two finishers will go head-to-head in late September, with the winner a heavy favorite against whoever emerges from the Democratic primary.

THE TRUMP FACTOR

For weeks, Strange and Brooks have lobbed attacks at one another as insufficiently committed to Trump.

Advertisements from supporters of Strange highlighted Brooks' endorsement of Texas Senator Ted Cruz for president last year, as well as criticism Brooks made of Trump at the time.

Brooks, a founder of the House of Representatives' far-right Freedom Caucus who survived the shooting attack on Congress members at a softball practice in June, has fired back by attacking Strange for his close ties to Senate leadership.

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Strange and Brooks have largely ignored Moore, though Strange backers have run some anti-Moore ads in recent days.

"I think both assume that Moore is going to make the runoff, and whoever gets to the runoff will likely beat him," Powell said.

Moore made his name in 2003 by refusing a federal court order to remove a Ten Commandments monument from the state Supreme Court building and losing his position.

After winning another term in 2013, he was suspended in 2016 and later resigned after directing state judges to defy the U.S. Supreme Court and continue to enforce a ban on same-sex marriage.

The former judge is popular among religious conservatives but had trouble expanding his base in previous campaigns for governor.

Each contender would likely serve as a reliable Republican vote in the Senate, with little daylight between their policy positions.

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"All three of these candidates are different intensities of the same flavor," said Steven Taylor, a political science professor at Troy University in Alabama.