Last Friday night, a small group of about five people were still hard at work preparing for the launch of former Democratic Gov. Brian Schweitzer's run for Montana's open Senate seat.

One person was editing a "folksy" video of Schweitzer talking about how "messed up" things are in Washington, D.C. One was emailing with Markos Moulitsas, the founder and publisher of the liberal Daily Kos, in an effort to splash out the message.

The rollout was supposed to happen on Monday.

On Saturday morning, Schweitzer announced that he would not run for Senate. It happened so swiftly that even the small team still plugging away on Friday found out about it just minutes before it happened.

"It shocked everyone," said one source involved in the planned rollout.

People from that team who were preparing Schweitzer's candidacy are blaming his sudden departure on Democratic infighting and bad blood among Schweitzer and the state's two current Democratic senators. And now, Democrats are worried that Schweitzer's exit from the race cost them their only chance in Montana — and could end up costing them control of the Senate.

The main target of their ire: U.S. Sen. Jon Tester (D). In the week before Schweitzer decided not to run, Democratic leaders began have doubts about Schweitzer's candidacy. Multiple sources close to Schweitzer pointed to Tester and his chief of staff directly as the source of that doubt — something that Tester's staff denies.

"It's already a big lift to ask Brian to leave his beautiful land in Montana and join a dim Senate where almost nothing gets done under current filibuster rules," one source directly familiar with Schweitzer's thinking told Business Insider.

The source said Tester was "sticking knives" in Schweitzer's potential run. In the days before Schweitzer decided to drop his potential bid, a number of stories emerged about Schweitzer's ties to "dark" and "secret money" groups.

Three sources close to Schweitzer's campaign fingered Tester's camp as the source of two unflattering stories that popped up in the past week — one in Politico and one in the local Great Falls Tribune. These sources singled out Tester and Tom Lopach, his chief of staff.

Tester's office vehemently denies that he had any role in Schweitzer's decision at all or in the stories.

"Jon Tester and his entire organization had nothing to do with the bad press that Brian Schweitzer got last week," said Andrea Helling, Tester's press secretary. "Jon Tester and his chief of staff have never provided a single anonymous quote attacking Brian Schweitzer."

The National Republican Senatorial Committee has also taken credit for building a mountain of opposition research on Schweitzer.

"We did our homework and there was a lot of rust under Schweitzer’s hood – a lot of rust," NRSC spokesman Brad Dayspring said last weekend.

The clash between Schweitzer and Tester's camps expose schisms in Montana's Democratic Party that have been prevalent for years. Schweitzer had a rocky relationship with Baucus during his two terms as governor, and he's also clashed with Tester, who has subtly accused him of not doing enough last year to get him re-elected. Tester and Schweitzer have not spoken since before the 2012 election.

Brian Schweitzer AP Schweitzer's belief that the stories came from Tester played a role in him dropping out of the race. According to a person familiar with his decision, he was not willing to endure a year and a half on the campaign trail with constant friendly fire — let alone the hits he'd be taking from Republicans eager to steal back a Senate seat.

"The Politico story was the tip of something that runs deep in Montana Democratic politics," said one source who was involved in Schweitzer's campaign rollout.

"Everybody was expecting a primary campaign with [Schweitzer] primarying Baucus. That's been expected for a couple years. The battle lines have been drawn for long time. ... But when Baucus dropped out, the assumption was that everybody would come together to hold his seat. In reality, people on the other side of that chasm realized that they'd be better off with Daines than Schweitzer."

"Daines" is Rep. Steve Daines, a Republican who is currently mulling a run. Now, those close to Schweitzer are privately resigned to losing the seat.

A Public Policy Polling survey taken in late June showed that Schweitzer was the only possible Democratic candidate who consistently beat most of the possible Republicans. Schweitzer was viewed favorably by 54% of Montanans, compared with just 40% who viewed him unfavorably.

They'll now have to turn to other candidates. One high-placed Democratic source said that the potential list includes EMILY's List President Stephanie Schriock, State Superintendent Denise Juneau, and Montana Supreme Court Justice Brian Morris.

For Schweitzer's team, at least, none of those candidates have much of a chance.

Said one source involved with Schweitzer's planned rollout: "Hopefully there will be accountability for people who torpedoed the campaign for the one person that could've held the seat."