Collins, a three-term veteran of the Senate, is running against Democrat Shenna Bellows in this year’s election.

During a Town Hall meeting at the Margaret Chase Smith Library, King emphasized the close relationship he’s developed with Collins during his two years in the Senate. He said often, senators from the same state are in a sort-of permanent competition. Not so with King and Collins, he said.

“I’m pretty sure we’re the only two senators with joint letterhead,” he joked.

But King and Collins have not always been political partners. In 1994, King defeated Collins in a four-person race to become Maine’s governor. Two years later, Collins first ran for — and won — election to the U.S. Senate. She easily won re-election bids in 2002 and 2008. In the latter race, she bucked a Democratic wave that rode Barack Obama into the White House and Democrats into congressional majorities by resoundingly defeating U.S. Rep. Tom Allen.

Fast forward to 2012, and King — by then a fondly remembered governor — was elected Maine’s junior senator, joining Collins to form Maine’s U.S. Senate team. The two serve together on the Senate Select Committee on Intelligence, and often issue joint statements.

On Friday, King joked that it had been easy to like Collins “once we got over the prickly part about 1994.”

Both senators on Friday said they respected each other’s eagerness to cross the often canyon-like partisan divide in Congress.

King said he had grown to admire and respect the senior senator based on their time together on the Intelligence Committee. He said the private nature of those meetings, which are not open to media, gave him a true picture of Collins.

“She’s a good senator, and I’ve seen it close-up,” he said. “She’s one of the most well-respected people in the Congress. … It’s not unusual to hear ‘I’m thinking about this bill, what’s Susan going to do?’ or ‘What are Susan’s thoughts on this?’”

Collins said she was “honored and delighted” to receive King’s endorsement, and said she hoped he would hit the campaign trail with her ahead of November’s election.

“Both of us worked so hard to try to bridge the partisan divide in the Senate that too often leads to excessive gridlock and incessant partisanship, and Angus is a problem-solver, and it has been great to work with him,” Collins said.

King caucuses with majority Democrats in the Senate, but he’s always linked his affinity for any party to political expediency. Caucusing with the majority ensures better committee assignments and closer proximity to power.

During the 2012 campaign, King said he might not caucus with either party in the Senate. After winning the election, he announced he’d caucus with Democrats because doing otherwise would “severely compromise” his effectiveness.

Earlier this year, he made national headlines after saying he was weighing his options on whether he’d caucus with Republicans if they won back a majority in the Senate this November. His staff later downplayed the comments, and King on Friday repeated that he’d make his decision about caucusing based on “what’s best for Maine’s people.”

Bellows said Friday morning that King called her Thursday evening to inform her that he would endorse Collins. She also was scheduled to appear Friday in Skowhegan, at a fundraiser being held in the home of King supporters, she said.

She said the endorsement amounts to little more than a bump in the road for her campaign.

“I respect Angus, and I wouldn’t have turned down his support, but what really matters are the Maine voters around the state — especially those who voted for Susan last time around, who are concerned that after the 18 years she’s been in office, we’re on the wrong track,” Bellows said.

The Democrat, and former chief of the Maine chapter of the American Civil Liberties Union, said she’d continue pointing out the differences between herself and Collins, namely the Republican’s record on civil liberties. Bellows has made opposition to NSA spying and drone warfare one of the keystones of her campaign — two areas she and Collins stand in stark contrast. She also has been vocal about her support for raising the minimum wage to $10.10 per hour, a position that Collins opposed.

She pointed to King’s defeat of not only Collins but former Democratic Gov. Joseph Brennan in 1994 as evidence that a relatively unknown candidate can take down a popular figure.

“We’re still six months out, and a lot can happen in six months,” said Bellows.

King’s past endorsements have crossed political lines. In 2010, King endorsed Democrat Chellie Pingree’s re-election bid in Maine’s 1st Congressional District. The same year, he endorsed independent Eliot Cutler, who lost the governor’s race by 2 percentage points to Republican Paul LePage.

As was the case with Pingree in 2010, King’s endorsement of Collins appears to be politically safe, as he is backing an incumbent with significant financial advantages and a lead in polling.

King said Friday he had not yet decided whether he will make an endorsement in this year’s gubernatorial election, which pits Cutler and LePage against Democrat U.S. Rep. Mike Michaud.

BDN State House bureau chief Christopher Cousins contributed to this report.

Sen. Angus King has endorsed Sen. Susan Collins, from porch of Margaret Chase Smith’s home in Skowhegan. #mepolitics pic.twitter.com/EdxIdE4bvJ

— Mario Moretto (@riocarmine) May 16, 2014

UPDATED 12:45 PM: SKOWHEGAN (AP) — Independent U.S. Sen. Angus King is endorsing his Republican colleague Susan Collins in her bid for a fourth term in the Senate.

At the Margaret Chase Smith Library in Skowhegan on Friday, King called Collins a “model U.S. senator” and praised her willingness to work across the aisle.

Collins is being challenged by a political newcomer, Democrat Shanna Bellows, former executive director of the Maine Civil Liberties Union.

King has not previously endorsed Collins. They were political opponents when he defeated her in a three-way gubernatorial race in 1994.

But the two have worked together well in Congress. Collins says she and King share a desire to bridge the partisan divide that often leads to gridlock in the Senate.

Bellow said Friday that she respects King’s thoughtful and independent decision.

UPDATED 6:35 AM: PORTLAND (AP) — Independent U.S. Sen. Angus King, who caucuses with Democrats and once considered Susan Collins a political foe, is endorsing his Republican colleague’s bid for a fourth term in the Senate, her campaign says.

The two were scheduled to appear together Friday at the Margaret Chase Smith Library in Skowhegan.

King has not previously endorsed Collins; they were political opponents when he beat her and Democrat Joe Brennan in a three-way race for governor in 1994.

But the two have worked well together in Congress. They created a “Gang of 14” to deal with the government shutdown, worked together on a student loan bill and combined their muscle to ensure that the Navy exercised an option to build an extra destroyer at Bath Iron Works.

“I think she’s one of the Senate’s MVPs — smart, tough, and always willing to listen. In my book, she’s a model senator and I’m delighted to support her re-election,” King told The Associated Press in a statement.

Collins is being challenged by a political newcomer, Democrat Shenna Bellows, the former executive director of the Maine Civil Liberties Union.

Bellows said Friday that it’s still early in the race and that voters are warming to her once they get to know her. “We’re still six months out, and a lot can happen in six months,” she said.

King, a former Democrat, campaigned on the idea of going to Washington to try to break political gridlock.

And Collins, one of only two congressional Republicans left in New England, has shown a willingness to work with lawmakers from both parties over the years.

During the government shutdown, the two joined forces with a “Common Sense Coalition” to press for a compromise to end the stalemate, reopen government and avert a Treasury default.

King praised Collins for attempting to forge a compromise despite the possibility that she could come under attack by tea party activists, as was former Sen. Olympia Snowe, another moderate Republican from Maine who retired because of what she called corrosive partisanship that left the Senate at a stalemate.

“She always puts Maine and the country first and isn’t afraid to cross party lines to get things done,” King said.

Collins, who faces write-in candidate Erick Bennett on the June 10 primary ballot, recently began stepping up her fundraising. In the last reporting period, she raised $870,000 and had more than $3.6 million in cash on hand. Bellows had $515,724 in cash on hand at the end of the reporting period on March 31.

The location chosen for Friday’s endorsement holds symbolic value for Collins. Collins has said Smith, the late senator who hailed from Skowhegan, was a mentor who inspired her.