Scott Walker presents a big, ripe political target for Democrats. So why haven't they been able to come up with a serious challenger to run against him?

Wisconsin's Republican governor has angered Democrats and many independents with his conservative stances on issues such as abortion, voting rights and, most of all, union-busting. Walker's approval ratings dipped perilously low as a result of his unsuccessful presidential bid. They have since ticked back up but remain subpar, in the mid-40s.

Still, Walker enjoys rock-solid support among Republicans. Wisconsin is a divided state, but Walker's fundraising prowess may keep him off the list of endangered incumbents next year.

"Democrats are really at a loss for what kind of person would even be competitive against Scott Walker," says Barry Burden, director of the Elections Research Center at the University of Wisconsin. "It may be some Democratic candidate we haven't thought of will run. But at the moment, it's dim."

Jared Leopold, communications director for the Democratic Governors Association, says the idea that Democrats will fail to challenge Walker seriously has been "overhyped."

"Walker's certainly vulnerable, and it's a state where we will find a strong candidate to run," Leopold says.

So far, though, Democrats are still looking. In the course of a single week last month, businessman Mark Bakken, former Green Bay Packers player Mark Tauscher and former state Sen. Tim Cullen all announced they would sit this election out.

By contrast, more than half a dozen credible Republicans are lining up to run against Democratic U.S. Sen Tammy Baldwin.

"One of the many downsides of having a party hollowed out at the state and local level is that it really saps the potential for people who can run statewide," says Paul Nolette, a political scientist at Marquette University.

The real danger for Democrats is not missing out on the chance to take out Walker but having a similar dynamic play out in other states. This should be a great cycle for the party, with Republicans having to defend 27 of the 38 governorships at stake this year and next.

But you can't beat something with nothing. Given the GOP's recent dominance at the state level, the Democratic bench is badly depleted in many states.

"Throughout a lot of these Midwestern states, the Democrats don't have a lot of obvious candidates or proven candidates," says Kyle Kondik, who track governors' races for the University of Virginia's Center for Politics. "You have a number of candidates who are in or are probably going to get in, but it's not a proven group, and none of them have good statewide name recognition."

Walker hasn't officially announced for re-election, but already some of the strongest potential Democrats, including Congressman Ron Kind, Kenosha County Executive Jim Kreuser and state Senate Minority Leader Jennifer Shilling, have taken themselves out of the running. The only Democrat in the race at this point is 25-year-old Bob Harlow, who ran unsuccessfully for Congress last year in California.

"Under normal circumstances, somebody as disliked as Walker by liberals, who has a 45 percent approval rating, would be attracting strong candidates," says Nolette. "But he isn't."

Wisconsin has traditionally been a competitive state. Wisconsin had voted Democratic for president in every election since 1984, until last fall. Republican Donald Trump carried the state by the narrowest of margins, less than 1 percentage point.

As recently as 2010, Democrats held both the governorship and both legislative chambers. But they have been shut out of power in recent years, with their legislative strength and much of their statewide vote pretty well confined to Milwaukee and Madison.

State supreme court races have been hotly contested, expensive affairs in the Walker years. Yet when conservative Justice Annette Ziegler came up for re-election on April 4, Democrats failed to field a challenger against her.

Given Trump's unpopularity nationwide, Democrats are optimistic about their chances to gain gubernatorial seats. But Walker has been able to prevail in tough environments before, including the 2012 recall election.

Walker, in fact, has won by similar margins in each of his three races for governor, in 2010, 2012 and 2014. In each, he took either 52 or 53 percent of the vote. Wisconsin is a badly polarized, narrowly divided state, but it appears to be tipped in the governor's favor.

"People have looked back at Walker's three elections and seen he's been successful in three different circumstances, and it's hard to see how it would be different now," says Burden.

Democrats know they'll be badly outspent. Walker has a national fundraising base he can draw on, built up when the Koch brothers and other conservatives sought to protect him after his move to strip public employee unions of collective bargaining rights triggered the recall effort. Walker's network has been enhanced by his presidential run and his position as head of the Republican Governors Association.

What's more, campaign finance restrictions have been weakened in the state. Walker survived allegations that his 2010 campaign had illegally coordinated with outside groups. And Walker and legislative Republicans killed the state's elections and ethics board in 2015, replacing it with weaker overseers.

"There's a huge challenge in raising the campaign dollars even to be remotely competitive with the millions and millions of out-of-state dollars that will flood Wisconsin to keep Gov. Walker in power," Cullen told reporters when he dropped out of the race last month.

It's certainly possible that Walker will yet be beaten. He was expected to be a formidable candidate for president, but his campaign dissolved two months after its official launch. Despite the usual Republican advantage in Wisconsin midterms, it looks quite possible at this point that Democrats will be more fired up than the GOP next year.

But at this early stage, Walker is looking strong enough to scare off most of his potential competition.

"To a certain extent, politics is risk management," says Mordecai Lee, a former legislator who teaches urban planning at the University of Wisconsin's Milwaukee campus. "The fact that so many people are not running shows they've done the risk management and decided it's not worth the risk."