HARRISBURG, Pa. -- To understand Pennsylvania's fast-changing political geography, look no further than Tom Killion.
After Democrats recently flipped six state Senate seats in the suburbs of Philadelphia and Pittsburgh, Killion is one of the chamber's last Republicans standing in those areas — and target No. 1 for Democrats in 2020.
That's when Pennsylvania will be a closely watched battleground in the presidential contest.
Killion's Delaware County-based seat has been held by Republicans going back to the 1800s, but President Donald Trump may complicate things for him next year.
"Have you been watching the elections?" Killion responded in an interview, when asked if his district was getting tougher to win.
For now, Killion is a chief sponsor of legislation that sounds like a progressive Democrat's wish list: reducing gun violence, reaching 100% renewable energy by 2050 and imposing a tax on natural gas production to underwrite a multibillion-dollar infrastructure package. That last policy point is atop Democratic Gov. Tom Wolf's agenda.
Located in Pennsylvania's southeastern corner, Killion's district is part of the heavily populated and politically moderate suburbs of Philadelphia. Once a bastion of Republican power, voter registration has shifted to favor Democrats over the past couple of decades, and Trump's election seemed to accelerate Republican losses and bolster Democrats' political activism there.
Killion has served in the Legislature since 2003 and has stood with Republicans on some of Harrisburg's most partisan bills. He has attained a 69% lifetime rating from the American Conservative Union, in line with other suburban Republicans in Pennsylvania's Senate.
Still, he said he has not shifted his legislative strategy to appear more liberal ahead of the 2020 election; he's always believed in working across party lines.
He is also used to winning with ticket-splitting voters. But he acknowledged that Trump has changed voter views in Philadelphia's suburbs since 2016.
"They're so angry at Trump that they're just pulling the straight D lever in the general election," Killion said.
In 2017, Republicans hemorrhaged local government seats in Philadelphia's suburbs.
Last year, the suburbs roared again , flipping three congressional seats outside Philadelphia and Pittsburgh to Democrats. Victories in state legislative races exceeded the expectations of Democratic strategists, and gave the party a majority of suburban Philadelphia's seats for the first time in modern history.
In Pittsburgh's suburbs, Democrats picked up another state Senate seat in a special election two weeks ago , helping shrink last year's 34-16 Republican advantage in the Senate to 28-22.
Those successes are raising Democrats' 2020 hopes of capturing the majority in a Senate controlled by Republicans for almost three decades. Pennsylvania's Senate and House are the last Republican-controlled legislative chambers in the northeastern United States.
Killion is, perhaps, the most vulnerable.
Last November, more than 60% of voters in his Republican-majority district backed Wolf and Democratic U.S. Sen. Bob Casey in their reelection bids .
In 2016, 55% of voters there backed Democrat Hillary Clinton over Trump. Still, Killion won his election that year, despite Trump's poor showing in Philadelphia's suburbs on his way to winning the state.
The state Senate's top Republican, President Pro Tempore Joe Scarnati of Jefferson County, dismisses Democrats' talk of winning a majority as just a fundraising strategy.
The suburbs, he said, are not lost to Republicans. They have seen swings in momentum before, he said, and will swing back — possibly in 2020 when Democrats must defend three suburban seats with Trump on the ballot.
"It's been real easy to beat up and criticize Donald Trump for a lot of reasons, because right now, Donald Trump has been running against himself," Scarnati said. "In 2020, there's going to be a Democratic candidate running against him and the people will have to decide: Do they want the extremism on the left or do they want Donald Trump back?"
Democratic Party leaders in Delaware County said they won't take anything for granted in challenging Killion. But Democrats also say voters aren't simply angry — they are sophisticated and know Killion is a vote for Senate Republican leaders who will block a progressive agenda.
"His first vote is for the Republican agenda and every vote after that is just for show," said David Marshall, executive director of the Senate Democrats' campaign arm.
For Killion, his election strategy will, to some extent, be the same: rely on people who know him and what he's done for the area going back to his days on county council in the 1990s.
But Killion suspects 2020 will also be different because of Trump.
"I don't know if it'll affect me because I've been doing this for so long, people know me," Killion said. "But you've got to worry."
Follow Marc Levy on Twitter at www.twitter.com/timelywriter .