In January of 2011, Senate Minority Leader Mitch McConnell called Senate Majority Leader Harry Reid a "friend."

"I think we trust each other, and I think that’s important for the institution," the Kentucky Republican told Bloomberg News.

Those comments came not long after McConnell had declared that the most important thing Republicans could do is defeat President Obama in 2012.

"I don’t think he really means that," Reid said on NPR. “I feel comfortable with my relationship with McConnell and the Republican caucus."

Fast forward to last night, when Reid declared that his Republican counterpart "tried to make love to the tea party and they didn’t like it."

Oomph. Actually, double oomph.

Reid's comments mark a new low in a relationship that has deteriorated gradually over the past few years -- and far more rapidly in the past few weeks. While they were never best buddies, Reid and McConnell were once able to work together to keep the Senate functioning. ("“They are as close as two people with limited social skills can be,” one Democratic aide told The Hill a few years ago.) While they would criticize each other's tactics, they would maintain Senate decorum.

"Historically, they both approached the Senate the same way," said former senator Judd Gregg of New Hampshire. "They're both people of the Senate, both masters of the floor, masters of the rules, and they understood that both sides had to be reasonably transparent with the other. They told each other what they were going to do, and they did it."

That's changed, and a combination of executive branch and 2014 politics is to blame. McConnell is up for reelection next year, and while he has a huge war chest and will almost certainly run a stellar campaign, polls suggest he is vulnerable. He faces not just Democrat Alison Lundergan Grimes but now a likely challenge from the right.

"I think McConnell's prime directive has been to avoid a credible tea party challenge," said Al Cross, the director of the Institute for Rural Journalism and Community Issues and a veteran McConnell-watcher for decades at the Louisville Courier-Journal. That goal has "made it difficult to operate in the way he would like to," added Cross.

Democrats see those political concerns as driving McConnell to go too far, especially in blocking a vote on several of President Obama's executive branch nominees.

"The final straw by all accounts was filibustering the executive branch nominations," said Jim Manley, a former longtime aide to Reid. "Throw in the concern that some of this is being dictated by Sen. McConnell's senatorial campaign, and Democrats have had it up to here."

Two weeks back, McConnell suggested that if Reid changed the Senate rules on filibusters he would go down in history as "the worst leader here ever.” McConnell's reelection campaign then tweeted out a picture that drove the point home:

But Republicans say the minority leader is only responding to Reid, who they think is becoming more partisan in part to damage McConnell's reelection prospects. The two senators have a longstanding, unspoken agreement that they will not campaign against each other, an attempt to avoid the acrimony that emerged after then-Majority Leader Bill Frist (R-Tenn.) campaigned for John Thune in his campaign against Minority Leader Tom Daschle (D-S.D.) in 2004.

"You have McConnell running for reelection and Reid's ratcheting up the rhetoric,
trying to get under McConnell's skin," said Ron Bonjean, a former Republican Senate leadership aide. "It's not in Senator Reid's interest to repair the relationship."

Despite the pact, many still point to the 2004 race Daschle-Thune race as a seminal moment in the decline of senatorial leader relations. "No matter what's said for political consumption there's no love lost," said Larry Forgy, a Kentucky Republican who campaigned for McConnell back in 1984 and went to law school with Reid. "It deteriorated with the defeat of Daschle." A super PAC that has Reid's blessing has launched an ad campaign against McConnell.

Whether it's in anyone's interest or not, both sides agree that despite the deal on some nominees, cordiality -- let alone friendship -- is a distant dream. On the plus side, as Manley says, "their relationship has nowhere to go but up."