Washington (CNN) Democratic Sen. Joe Manchin, who's facing a tough re-election bid in West Virginia, urged his colleagues on Tuesday to pledge not to campaign against other sitting senators.

"I don't see anybody in public service that's willing to put their name on the ballot as my enemy. If you're willing to serve, then I'm your comrade. I'm willing to work with you," Manchin said on the floor just before he signed a large, chart-sized pledge.

The practice has created a "hostile environment," Manchin told CNN before his speech, because senators remember which of their colleagues tried to defeat them. "It could be from five or 10 years ago. But they haven't forgotten."

The practice has created a "hostile environment," Manchin said before his speech in an interview with CNN, because senators remember which of their colleagues tried to defeat them. "It could be from five or 10 years ago," he continued. "But they haven't forgotten."

It's unclear how senators will respond to the pledge, though Manchin acknowledges it's unlikely he'll get droves of support in what he called the current "toxic environment."

"If I get one other person, that'd be a big win," he said.

Sen. John Cornyn, the No. 2 Republican in the Senate, mentioned Manchin's pledge on Twitter and appeared to ask the No. 1 Democrat in the Senate, Minority Leader Chuck Schumer, for his thoughts.

"Sen Schumer?" his Twitter said.

The non-binding pledge addresses five actions, according to a copy of the document provided to CNN. Anyone who signs it would be promising (1) not to campaign against sitting colleagues; (2) not to directly fundraise against them; (3) not to distribute any direct mail against them; (4) not to appear in or endorse any ads directed at them and (5) not to use or endorse social media campaigns that attack them.

The fundraising aspect, Manchin said, also applies to raising money for Senate campaign groups, like the National Republican Senate Committee and the Democratic Senatorial Campaign Committee, which work to defend and pick up seats for their respective parties, even if that means ousting incumbents.

Manchin stressed the pledge would only cover races with a sitting senator, not open Senate races. So, for example, if Democratic Sen. Pat Leahy of Vermont signed the pledge, he could still campaign for the Democratic candidate in the Senate race in Arizona -- where Republican Sen. Jeff Flake isn't running for re-election -- but Leahy couldn't campaign for the Democratic candidate in Nevada, where Republican Sen. Dean Heller is seeking to keep his seat.

A filing with the Federal Election Commission, however, shows that Manchin's political action committee, Country Roads, donated $1,000 to Democratic candidate Alison Lundergan Grimes, who challenged Senate Majority Leader Mitch McConnell in Kentucky in 2014.

Asked to explain, Manchin's' spokesman Jonathan Kott said Manchin at the time would "only donate against the two senators who are most responsible for the dysfunction in Washington -- Senator McConnell and Senator Reid" (Reid, a Democrat, was the Senate majority leader at the time. FEC records do not show any contributions from Manchin's PAC for or against Reid).

Moving forward, Kott said, Manchin would not donate against the two current Senate leaders, McConnell and Schumer.

The pledge is part of a larger effort Manchin is expected to push in the coming weeks and months as he seeks to sharpen a brand of bipartisanship and civility ahead of the midterm elections. While Manchin was elected in 2012 with 61% of the vote, Republicans view West Virginia as a potential pickup seat to help widen their slim 51-49 majority in the Senate. President Donald Trump won the state with 68% of the vote in 2016, and the state's newly elected governor left the Democratic Party in August to become a Republican.

Manchin is widely considered the most conservative Democrat and sided with Republicans on key votes last year, such as confirming Supreme Court nominee Neil Gorsuch and Attorney General Jeff Sessions. But he has faced heavy criticism from Trump for voting against the Republican-backed tax reform plan in December and not supporting efforts to repeal and replace the Affordable Care Act.

"I like Joe," Trump said in a New York Times interview . "You know, it's like he's the great centrist. But he's really not a centrist. And I think the people of West Virginia will see that. He not a centrist."

Attempting to navigate the political battle ahead in November, Manchin has taken on a new message of late that Washington "sucks" -- including in his response to Pence last week -- but he's still determined to run for re-election.

Campaigning for the defeat of colleagues was largely unheard of until the past two decades and has even been practiced among the highest ranks of the Senate. In 2004, then-Senate Majority Leader Bill Frist, R-Tennessee, traveled to South Dakota to campaign for the opponent of the then-Senate Minority Leader Tom Daschle. It was an unprecedented move for a Senate majority leader but it demonstrated the competitive, cutthroat political climate.

Others have made it a personal commitment to not do so, like Sen. Angus King, an independent from Maine who caucuses with Democrats. "I don't know how you're expected to work together with somebody you know that tried like hell to kick you out six months before," King said in a podcast he did with Manchin last month.

When Sen. Susan Collins was new to the Senate in 1997, the late Republican Sen. John Chafee advised her to never "campaign against those with whom (she) served," a principle that was known as the "Chafee rule," she wrote in a 2011 op-ed.

"The Senate is too small a place for that, he counseled. Campaign for your fellow Republicans and go to states with open seats, but do not campaign against your Democratic colleagues. It will poison your relationship with them," she wrote, recalling Chafee's advice.

Ultimately, Manchin said he'd like to see the Senate make it an ethics violation to campaign or raise money for the ousting of incumbents. Asked if he had concerns that such a penalty could violate free speech rights, Manchin said, "We work under our own rules and regulations here in the Senate."

"It's not violating the Constitution," he added. "If you're going to be a senator, that would be the ethics you agree to."

This story has been updated to include information about contributions from Manchin's political action committee in previous elections and developments through the day Tuesday.