Alabama Secretary of State John Merrill increased his turnout estimate for Tuesday's Roy Moore-Luther Strange showdown because of President Trump's involvement in the race, but only slightly.

John Bennett, press secretary for Merrill, said the estimate has gone from 12 percent to 12 to 15 percent.

Former state Supreme Court Chief Justice Moore and Sen. Strange square off in Tuesday's runoff for the Republican nomination for the Senate seat vacated when Jeff Sessions became attorney general. Strange has held the seat on an interim basis since his appointment by former Gov. Robert Bentley in February.

The Moore-Strange winner faces Democratic nominee Doug Jones on Dec. 12.

Turnout for the Aug. 15 primary was 18 percent, with 589,175 voters going to the polls, according to unofficial results from the secretary of state's office. Of those, 72 percent voted in the Republican primary.

Moore led a nine-candidate field in the primary with 164,524 votes, 39 percent, to 138,971 for Sen. Strange, 33 percent.

Jones trounced a seven-candidate Democratic field, getting 66 percent of the vote to win the nomination without a runoff. Jones got 109,105 votes, fewer than Moore or Strange.

President Trump, who endorsed Strange before the Aug. 15 primary, backed up that endorsement with a campaign speech for Strange on Friday night in Huntsville. Vice President Mike Pence is visiting Alabama to campaign for Strange today. A super political action committee controlled by the Senate Majority Leader Mitch McConnell has spent heavily on television ads attacking Moore.

Moore has drawn some big-name supporters, too, including former vice presidential candidate Sarah Palin, who led a rally in Montgomery on Thursday night, former Trump adviser Steve Bannon, and others.

A poll released Monday showed Moore leading with 52 percent, Strange at 41 percent and 7 percent undecided.

Richard Fording, a political science professor at the University of Alabama, said the split among national Republican figures on the Alabama race could reduce turnout.

"Mitch McConnell's involvement in backing Luther Strange has undoubtedly resulted in some degree of backlash due to the fact that he is tied to the Washington 'swamp,'" Fording said in an email. "However, the backlash is at least partially cancelled out, I think, by Trump's endorsement of Strange. This has no doubt left some voters confused as to whether Strange is truly a member of the establishment. These conflicting signals may very well serve to depress turnout (above and beyond what would normally be low turnout, anyway)."

Fording said there are probably several reasons Moore is ahead in the polls.

"If I had to guess, the bigger factors driving Roy Moore's lead in the polls leading up to the race would be (1) his name recognition and association with Christian values, (2) his history of standing up to the federal government, (3) his perceived integrity for doing so. Also, I believe that Strange's appointment to the Senate by our unpopular governor has been interpreted by many voters as kind of a sleazy deal to ease the pressure on the investigation into Governor Bentley's ethics violations.

"As for turnout effects, I would expect that Moore will have a decided advantage due to his popularity among Evangelical Christians, who often turn out at higher rates than other voters."

A new state law that bans crossover voting in party primary runoffs will be in effect for the first time Tuesday.

Voters who voted in the Democratic primary on Aug. 15 will not be allowed to vote in the Republican party runoff.

The law makes no changes on how people can vote in general elections.