WASHINGTON -- Who wants to run against Bob Casey?

Republicans energized by 2016 victories in the Keystone State hope to keep momentum rolling next year against the Democratic U.S. senator from Scranton. But so far few challengers have stepped forward, and a volatile political environment has built a thicket of uncertainties for anyone considering a bid to unseat one of the most recognizable names in Pennsylvania.

Republican insiders wonder if anyone can replicate President Trump's unexpected win in the state. They are already seeing signs that the president and fellow Republicans might face a fierce backlash -- his early moves have inspired a wave of activism on the left, reminiscent of the 2010 tea party fury against President Barack Obama. Top Pennsylvania candidates may opt to run for governor.

And any Casey challenger will have to raise huge sums of money. Last year's Senate race in Pennsylvania cost nearly $180 million.

Still, Republicans say they see a good chance to go after Casey, one of 10 Senate Democrats up for reelection in states that Trump won. The GOP is hoping to expand its two-seat Senate majority -- giving it more breathing room to advance Trump's agenda -- while Democrats hope to boost their numbers and resistance.

"We start out thinking that Senator Casey is vulnerable, especially thinking of the way the state went both for the president" and for Sen. Pat Toomey, said newly elected Republican state chairman Val DiGiorgio. "His record is far too left-wing for Pennsylvania."

Casey allies believe he's in a strong position, especially because of his historical support from the kind of blue-collar Democrats who chose Trump last year over Hillary Clinton. The senator seems to be banking on the traditional midterm repudiation of the sitting president, visibly and vocally opposing Trump at every opportunity.

Only one well-known challenger, State Rep. Rick Saccone, of Allegheny County, has filed federal paperwork to run. The outspoken conservative pledged to help advance Trump's agenda.

"We've elected a new president who I think is going to put [the country] on the right track, but he needs a lot of help," Saccone said in a telephone interview Thursday.

The names of other potential candidates also emerged in conversations with Pennsylvania GOP insiders, though few had made plans, and in some cases it was unclear if the individuals were actually interested or if party leaders were simply hoping to persuade them to consider a bid.

One, Jeff Bartos, a Montgomery County real estate developer, political fund-raiser, and GOP committeeman, is said to have met with national party officials to talk about what could be his first-ever run for public office.

"We all deserve better from Washington," Bartos said in an email. "I am seriously taking a look at whether I can help get things moving in the right direction again."

Others have encouraged state House Majority Leader Dave Reed, of Indiana County, to run, said longtime party power broker Bob Asher.

Reed has created a slick campaign-style website, without specifying its purpose. He "wants to run statewide," DiGiorgio said. But Reed has not indicated he'll jump into the Senate race.

Steve Miskin, a spokesman for Pennsylvania House Republicans, said Friday that Reed has been approached by "a number of people gauging his interest about a number of different positions, including the U.S. Senate, but right now he is totally focused on this legislative session."

He has declined, however, to rule out a future run for another office.

Ambitious Republicans have two options, because Gov. Wolf is up for reelection, too, and considered vulnerable. Jockeying for the Senate contest may have to wait for the governor's race to shake out.

Party insiders also said Jim Cawley, the Corbett administration lieutenant governor from Bucks County, remains popular within the establishment and is seen as a potential statewide candidate. Cawley did not return calls seeking comment.

The state's 13-strong House delegation might seem a likely place for recruiting, but they also have incentives to stay put. With the GOP controlling Congress and the White House, sitting representatives have a chance to play roles in major legislation without the risk of giving up their seats to run against an incumbent with a deep history in Pennsylvania.

Rep. Pat Meehan, of Delaware County, floated the idea of running but passed, saying he felt he had a better chance to get something done on the House's powerful Ways and Means Committee.

Several major variables weigh on anyone considering a run.

Republicans may be hard-pressed to duplicate the support Trump drew with his distinct style and message.

"It went that way because of a number of factors that might not be there again," including the intense disdain toward Clinton, said Manny Stamatakis, a top Republican fund-raiser. He and others said the mild-mannered Casey has not engendered anywhere near that level of opposition.

Money is another obstacle.

Last year's race between Toomey and Democrat Katie McGinty produced more than $177 million in campaign spending, including from independent political groups and during a competitive Democratic primary. Toomey raised and spent nearly $31 million.

"It's a daunting task to take two years out of your life and spend it raising money, but the rewards are tremendous," said Rob Gleason, the recently retired state GOP chairman.

Casey, though, is not a prolific fund-raiser. He had $1.6 million in campaign funds as of Dec. 31. At the same stage in his reelection cycle, Toomey had $5.8 million.

In addition, GOP challengers might not have the same level of outside support McGinty enjoyed last year, when Pennsylvania was one of the four premier Senate races in the country. Republicans may have more tempting 2018 targets in conservative states such as North Dakota, Montana, West Virginia, and Indiana.

At least one third-party candidate says he's running: Libertarian Dale Kerns, of Delaware County, who has set up a website and declared his intentions, though he has not filed federal paperwork. A Williamsport resident, Paul DeLong, has also filed to run in the Republican primary. He also filed as a candidate for the 2004 Senate race, but there is little evidence he did any serious campaigning.

Casey, for his part, has mapped out a clear strategy with his aggressive posture against Trump. He appears to be banking on running against the president -- no matter whose name winds up on the actual ballot.