Incumbency is a huge advantage for politicians seeking reelection. It's less of an advantage for those who become incumbents by accident.

As many as four governors will be running this year after rising to the office because their predecessors left early. None is getting a free ride. All of them are being challenged in primaries.

When a lieutenant governor becomes governor late in the term, she has limited time to establish an agenda or identity of her own. Meanwhile, other candidates who'd been eyeing the job may already be geared up and ready to go.

"Other candidates have been waiting, and they're not going to step aside just because someone got the post late in the second term of a sitting governor," says Geoffrey Skelley, a political analyst with the University of Virginia Center for Politics.

President Trump appointed three Republican governors to ambassadorships last year -- Terry Branstad of Iowa, Nikki Haley of South Carolina and Sam Brownback of Kansas. Brownback's confirmation has stalled in the Senate, but he's term-limited this year anyway and his lieutenant governor, Jeff Colyer, is running to replace him.

In addition, Kay Ivey became governor of Alabama last April when Robert Bentley resigned in the midst of a sex scandal.

All four of these states favor Republicans, which means the GOP nomination is too valuable to simply concede to the newcomer who was lucky enough to replace the governor.

"When you have a large Republican bench in places like South Carolina and Alabama and Kansas, that makes it less likely for there to be an inclination to defer," Skelley says.

Trump and his team were canny enough to pick governors from states where their seats would remain fairly safe for the party in their absence. That wasn't always the case under President Obama, who picked red-state Democratic governors such as Kathleen Sebelius of Kansas and Janet Napolitano of Arizona, whose jobs soon fell to Republicans.

"Typically, there are advisers in the White House who do their best to make sure that any appointees are from states where they're not going to lose the seat the next time around," says Marjorie Hershey, a political scientist at Indiana University. "Obama didn't always get the memo on that one."

All told, since World War II a total of 62 "replacement" governors have sought terms of their own after ascending to the office due to vacancies, by Skelley's count. Only 63 percent of them went on to win -- a lower success rate than that enjoyed by incumbent governors as a whole. Fourteen of them lost in general election contests, while nine were defeated in primaries.

"A lot of people don't see them as legitimate candidates," says Darryl Paulson, an emeritus government professor at the University of South Florida-St. Petersburg. "We didn't elect them; they were selected to fill the interim."

As a case in point, a poll released last month showed that 51 percent of Iowans approve of Kim Reynolds' performance as governor. The same poll, however, showed that 49 percent of voters in the state would prefer to see someone new in the job.

Reynolds is a Branstad protege. She was being groomed to replace him but has never won statewide office on her own. Lieutenant governors and other successors who have won statewide office on their own, as opposed to having been elected as part of a ticket, typically fare better, Skelley says. They have their own profile and a proven ability to raise money and attract votes.

The current crop of replacement governors hasn't had much time to establish their own records. At this point, they've had a year or less on the job.

In the case of Colyer in Kansas, he's still waiting to be handed the baton. With Brownback among the nation's least popular governors, Colyer was going to struggle whether running as the sitting governor or the lieutenant governor.

"The only way for him to differentiate himself would be to run as not-Brownback," says Michael Smith, who chairs the political science department at Emporia State University.

The Republican field is large, with Secretary of State Kris Kobach generally considered the frontrunner. But fundraising figures released last week show that Colyer is well ahead by that measure. He raised $632,000 last year, compared with $355,000 for Kobach.

In contrast with Colyer, Ivey enjoys a sort of anti-incumbency advantage, having replaced a disgraced predecessor.

"After Robert Bentley, any governor's going to look good who doesn't make any huge mistakes, and Kay Ivey has made very few mistakes," says Bill Britt, editor of the Alabama Political Reporter, an online news site.

Nevertheless, Ivey faces a half-dozen GOP competitors, including Huntsville Mayor Tommy Battle, state Sen. Bill Hightower and Agriculture Commissioner John McMillan.

"Battle was not anticipating Kay Ivey running, but had already made up his mind and had his backers," Britt says.

In South Carolina, Gov. Henry McMaster faces several challengers, including Lt. Gov. Kevin Bryant and Catherine Templeton, a former state health department director under Haley. Templeton has proven to be an able fundraiser, but a Mason-Dixon poll released last month showed McMaster well ahead of the field, with support from 51 percent of Republicans surveyed.

Although Reynolds remains politically in Branstad's shadow to some extent, she's got the backing of most of his donors and the party establishment in general. Cedar Rapids Mayor Ron Corbett, a former state House speaker, is challenging her in the Republican primary, but his campaign has yet to take off.

"The general opinion seems to be that he's going to get clobbered," says Dennis Goldford, a political scientist at Drake University.

At this stage, despite the challenges, most of this year's replacement governors look like good bets to win their party's nomination. They'll still face contests in the fall and Democrats are looking more active in these races than is generally the case in these states. 

But all four are in states where Republicans are generally favored, which makes the party's nomination that much more valuable.

"The Republican nomination is a plum position, because you're in a good position to win," political scientist Gibbs Knotts of the College of Charleston says about the South Carolina race. "If you can win that nomination, you have very good chances of being elected governor. It only  comes up every eight years, so other politicians have their eyes on it."