Pennsylvania U.S. Senator Bob Casey was supposed to attend a fundraiser in Pittsburgh last Friday. Instead, the Scranton Democrat joined his Lehigh Valley Republican U.S. Senate colleague Pat Toomey for the somber memorial service of Christopher David Hill, a U.S. Marshal killed in Harrisburg while serving a warrant to a fugitive.

Hill was an 11-year veteran of the U.S. Marshals Service and an Army veteran. He died when executing a warrant for the arrest of Shayla Lynette Towles Pierce, who was wanted by the Harrisburg police for terroristic threat offenses, when 31-year-old Kevin Sturgis of Philadelphia opened fire on the officers from the second floor of her home.

During last week’s brief but politically-charged shutdown, Casey was passionate in his opposition to Republicans, vowing to not ‘genuflect’ to their demands. Never once, though, did he attack Toomey personally.

For his part, Toomey, has never once attacked Casey personally, even though the Scranton Democrat is up for re-election this year and is potentially vulnerable against Republican Rep. Lou Barletta.

In fact, in 2016, when Toomey was in a tight race against Democrat Katie McGinty, it is hard to find one instance where Casey blasted his colleague when campaigning for McGinty or for Hillary Clinton.

This is a notable relationship.

Casey and Toomey are one of only 12 pairs of U.S. Senators who represent the same state but come from opposite parties. Casey and Toomey, for the most part, have adopted a ‘no fire’ rule against each other, even during election year cycles.

The opposite-party pairs include Ohio’s Rob Portman and Democrat Sherrod Brown, Colorado’s Republican Cory Gardner and Democrat Michael Bennet, Florida's Marco Rubio and Bill Nelson, West Virginia's Joe Manchin and Shelley Moore Capito, and Wisconsin has Ron Johnson and Democrat Tammy Baldwin.

While all 12 sets sort of cancel each other out on big votes, typically splitting their votes along party lines, they all do tend to work together well nominating and confirming federal district court nominees, something Casey and Toomey have excelled at, no matter which man has been in the minority at the time.

Of all of them, the only pair who does not hold their fire is Wisconsin’s Johnson and Baldwin.

Case in point, last week during the shutdown, not only did Johnson fire off a statement saying, “Unfortunately, Senator Baldwin and 43 of her colleagues decided to play politics with people’s lives by refusing to fund health care for vulnerable children, support for the finest among us serving in the military, and the rest of government,” he also jabbed her on Twitter on his official Senate account several times.

And don’t think the favor goes unmet. Baldwin consistently returns fire.

Baldwin and Casey are two of several incumbent Democrat U.S. Senators up for re-election this year in split states that went for Trump in 2016, including West Virginia, Ohio, Pennsylvania, and Montana.

To date, Baldwin is the only one personally taking heat from her colleague. The question is why? All of the other pairs nearly always cancel each other out on partisan votes, but always find a way to work together on non-controversial matters such as federal regulations, securing funding for large military and federal installations in their states, and sometimes handling constituent visits by hosting joint breakfasts for larger groups who travel to Washington to visit.

So, what makes their relationship so brittle? And will that help in an election year? If it does, who does it help?

Part of their disconnect is that Baldwin is from Madison, a very political town, and she has been involved in politics since she was in law school. Johnson was a very successful businessman before running for office. He's not from Madison, and if you know anything about Wisconsin, that says everything.

She is also very far left, and he is far right. She won the Wisconsin Senate seat over former governor Tommy Thompson in a nasty, expensive race by four percentage points in 2012. He held his seat in a dramatic come-from-behind race in 2016 over Russ Feingold, the former Democratic senator he initially unseated in 2010.

Wisconsin is a changing state politically, and Baldwin is one of only two statewide elected Democrats currently in office there. Republicans hold the state house, the state Senate, the majority in the congressional delegation, and every swing congressional seat.

Despite Baldwin’s left leanings, she did take a scathing hit from MoveOn.org, with the liberal group berating her for her vote last Monday to reopen the federal government without including any deal to protect Dreamers.

And Johnson has taken the lead to keep Wisconsin Republicans united ahead of the August Republican primary contest between businessman Kevin Nicholson and state Sen. Leah Vukmir. Johnson has pushed both candidates to sign a united pledge.

Despite heading into the second year of his second term, Johnson has always done things outside the norm of political procedures, because he has never shed his outsider persona. He still is that business guy who happened to become a U.S. Senator.

This is one of the Senate races going under the radar of the national press, but Wisconsin Republicans should not be taken for granted. In the past 8 years, they have fought back and won races they in theory had no business winning in the first place, and both Johnson and Scott Walker, the GOP governor who is seeking his third term, have built formidable grassroots operations ahead of both Walker's race and whoever ends up becoming Baldwin’s challenger.

Look for Johnson to have a lot to say ahead of this race — something you won’t likely see from his other Republican colleagues in Montana, West Virginia, and Ohio. But that’s the norm for Johnson, who always works outside the system — and typically prevails.

Salena Zito is a columnist for the Washington Examiner.