Josh Hawley, the Missouri attorney general, has emerged as the favorite Republican choice to challenge Democratic Sen. Claire McCaskill in 2018.

Republican insiders said Monday that Rep. Ann Wagner's decision not to run for Senate opens the door for the 37-year-old Hawley, who has been telling associates that he wasn't planning a bid. The constitutional conservative with an Ivy League pedigree has sparked the interest of influential Missouri Republicans after winning more votes on the 2016 ballot than any other GOP candidate on the statewide ballot.

"He's a conservative dream candidate. He is young, energetic, attractive, an outsider — a Federalist Society down the line intellectual and to an extent a grassroots conservative. He's a [Utah Sen.] Mike Lee candidate, but better looking," a Missouri GOP insider told the Washington Examiner, on condition of anonymity so as not to publicly favor Hawley over other Republicans who might seek the nomination.

Wagner was recruited by Republican leadership because of her presumed ability to neutralize McCaskill with women and avoid a repeat of 2012, when the senator, vulnerable then as she is now, survived re-election because her GOP opponent, then-Rep. Todd Akin, was so poorly received by women.

But some Republicans say Hawley has gained currency with Republicans in the past few months, quietly replacing Wagner as the preferred recruit of Senate Majority Leader Mitch McConnell and the NRSC, the party's Senate campaign arm, because he doesn't have a voting record to dissect and might easily unify the GOP base.

In early April, Josh Holmes, McConnell's confidant and former chief of staff, called attention on Twitter to a Kansas City Star story about prominent Republicans urging Hawley to run for Senate.

Wagner is a wealthy St. Louis native who didn't immediately embrace President Trump after he secured the GOP nomination last year, There was concern that McCaskill, a vulnerable but wiley Democrat from Kansas City, might have been able to drive a wedge between Wagner and culturally conservative Trump enthusiasts in exurban and rural Missouri.

"Hawley emerged as the No. 1 draft pick in early spring because he unites the entire Republican base," a Republican strategist in Washington said.

John Hancock, a former chairman of the Missouri Republican Party and longtime political adviser to Wagner, disputed suggestions that the congresswoman was weaker than other potential GOP candidates outside of the suburbs. "She would have been a great candidate. A really good matchup with McCaskill," he said.

Democrats are vowing a spirited challenge regardless of the nominee, charging that Trump is proving to be a drag on the down ticket, although there's no evidence of that in Missouri, where his approval ratings are in good shape — much higher than the national average of around 40 percent.

"Congresswoman Wagner reached the conclusion voters already know: nobody can out-work Sen. McCaskill and no one else would be a more effective champion for the state's working families," David Bergstein, spokesman for the Democratic Senatorial Campaign Committee, said in a statement. "The spotlight will now shift to other potential candidates who have been hiding from the reckless Republican agenda, but for these potential candidates there will be no escape from the GOP's proposals that will hurt Missouri's middle class."

Hawley could have problems of his own if he runs.

Republican candidates that might have deferred to Wagner, active in Missouri politics for 30 years, might jump into the GOP primary. Hawley's young age and short time on the political stage could lead other, more established potential Senate candidates, to take a look at the race. They include Rep. Blaine Luetkemeyer, who could seed a campaign with a couple of million dollars of his own money; state Treasurer Eric Schmitt, who has an experienced political team around him, and state House Speaker Todd Richardson.

Then there's Hawley's criticism of career politicians, made in his opening and closing television ads for his 2016 attorney general's race.

"Jefferson City is full of career politicians just climbing the ladder, using one office to get another. I'm Josh Hawley; I think you deserve better," Hawley says in the spot, surrounded by men in suits climbing ladders.

Hawley would have to explain why he was having a change of heart and launching a Senate bid less than a year after he won his first term as attorney general. A Republican strategist who has consulted on races in Missouri said Hawley's 2016 campaign wasn't the difficult test that the 2018 Senate race could be, even against a Democrat as vulnerable as McCaskill.

"The case against Hawley is, he's never had to prove himself. He's got this golden-boy glow from afar," the GOP strategist said.