I bumped into Karen Simpson in Frederick. We were at an outdoor reception for a conference on Maryland main streets, enjoying what felt like the first cool night of fall.

Six years ago, she was a long-shot Democrat running for the House of Delegates in ruby-red Pasadena. Predictably, she lost. Then her husband got a job as a pastor in Frederick, and rather than waiting for retirement to join him — the city had always been their dream — friends pointed out an opportunity.

Del. Karen Lewis Young was running for the state Senate in Frederick, aiming for the seat opened by the retirement of her husband, Sen. Ronald Young. She won, and Simpson was elected to replace her in 2022.

“I must have knocked on 2,000 doors,” she said.

Today, Simpson’s knocking on hundreds more doors for Democrat Angela Alsobrooks, the Prince George’s County executive seeking to replace U.S. Sen. Ben Cardin. Her opponent, Republican Larry Hogan, does well outside the Baltimore-Washington corridor, so Frederick should be his to win.

But, in the 18 years since Cardin was first elected to the Senate, this community on the edge of the Blue Ridge Mountains has changed. It is the potential key to victory in a race that could determine control of the Senate.

“It really has changed a lot,” said Kai Hagen, a former county councilmember who lost the Democratic primary for county executive two years ago. “The community has gotten much bluer.”

The Senate race will depend on turnout across Maryland. Democrats are motivated by the presidential contest, but Hogan made friends among conservative Democrats and independents during his two terms in the State House.

Montgomery and Prince George’s counties will join solid-blue Howard County and Baltimore in going for Alsobrooks — and that’s more than 1 million potential votes from registered Democrats alone.

Hogan will take Anne Arundel, his home county, and neighboring Baltimore County. The results will be close, and — unlike his successful runs for governor — not enough to tip the scales when combined with his support in the smaller counties in the western and southern parts of the state and on the Eastern Shore.

That means Frederick County and its 200,000 voters are poised to play a deciding role.

It wasn’t this way when Paul Sarbanes retired in 2006. Cardin, then a five-term congressman from Baltimore, faced then-Lt. Gov. Michael Steele and won statewide by 10 percentage points. Conservative Frederick County went the other way, voting for Steele by more than 30 points.

Ten years later, U.S. Rep. Chris Van Hollen clobbered Republican Del. Kathy Szeliga to replace retiring Baltimore Democrat Barbara Mikulski. He took 60% statewide but — like Cardin before him — lost Frederick. Only this time, Van Hollen cut the margin in half — to 5 points.

The difference was simple. People — lots more of them.

The population has more than tripled since 1970. Between Cardin’s first Senate win 18 years ago and this year, it added 71,000 new residents. It’s as if the entire population of Glen Burnie relocated.

“The people who have been in the county for a long time are overwhelmingly conservative,” said former Republican state Sen. Mike Hough, who lost the race for county executive to Democrat Jessica Fitzwater in 2022. “The people who are moving in are the Democrats.”

They came to enjoy the happy accident of life in Frederick County.

The 20th-century sprawl that transformed Maryland by the 1980s didn’t make it to Frederick. That gave civic leaders such as Democrats Ronald Young, who served as mayor of Frederick for 14 years, and Hagen time to prepare for its inevitable arrival.

They invested in schools and parks. They protected farms and the county seat, a city founded 100 years after Annapolis, from becoming another Silver Spring or Columbia.

Places like Urbana to the south absorbed thousands of new residents, while towns like Thurmont in the north kept their rural character. Historic buildings in downtown Frederick filled with restaurants, shops and other amenities.

“I’m always amazed at how many people I meet who came here because they were visiting a friend or came here for a weekend and fell in love,” Hagen said.

That includes me. Sitting atop a double-decker bus, I listened as the guide described what we were seeing. “Beautiful” buildings. “Beautiful” homes. “Beautiful” Baker Park. The “beautiful” sound of its “beautiful” 49-bell carillon.

“Our city arborists do a great job of trimming these beautiful trees to 15 feet so we can pass beneath,” she said.

That’s when the soft tips of a low-hanging branch smacked me in the face, but she was right. It is a beautiful city, easy to love, even if not everyone loves the changes.

“It feels like it’s growing too much,” said the bartender at Olde Mother Brewing, one of three brewpubs in the city. “I moved here from Pennsylvania, and it seems like it’s all moving so fast.”

Longtime residents tend to be anti-abortion, are passionate about gun rights, worry about transgender people and fear undocumented immigrants — the county sheriff’s office has made millions from a federal program that pays local law enforcement to detain them.

Newer people tend to support abortion rights, champion LGBTQ representation and demand humane treatment of immigrants — Frederick expanded voting in municipal elections last month to all adult residents regardless of citizenship.

Now, Democrats outnumber Republicans countywide, with independents close behind. The split is close. You can see it in the 6th District congressional race, where Democrat April McClain-Delaney and Republican Neil Parrott argue over what it means to be local.

“It used to be the Republicans pushing the growth,” Hough said. “Now they’re the ones trying to slow it down.”

Into all this come Alsobrooks and Hogan.

Their campaigns here are the same they’re running elsewhere.

“He is handpicked by [Senate Minority Leader] Mitch McConnell; he was endorsed by Donald Trump because they share one goal in mind: that is to grow a majority in the Senate and to control the agenda in the way I believe will take away freedom,” Alsobrooks told radio station WFMD in July.

Two weeks later, Hogan wrote in an op-ed for the Frederick News-Post: “Instead of pointing fingers and playing politics, I’ll reach across the aisle and work together to make life more affordable for Marylanders, just like I did for eight years in Annapolis.”

In the end, Frederick County’s math will decide the Senate race. More than 46,000 people have moved there in the decade since Van Hollen narrowly lost it.

Most of them, the numbers show, vote Democratic.