Former Maine Gov. Angus KingAngus KingCollins says running as Independent 'crossed my mind' Susan Collins and the American legacy Coordinated federal leadership is needed for recovery of US travel and tourism MORE — an independent — will announce a run for retiring Sen. Olympia Snowe's (R-Maine) seat in the Senate.

Two sources close to King told the Kennebec Journal he would announce his campaign Monday night. King did not respond to messages seeking confirmation.

A popular two-term governor and professor at Bowdoin College, King has a history of working with both parties and a demonstrated appeal to the state's large swath of independent voters. About 390,000 registered voters in Maine belong to neither major party — more than are registered as either Democrats or Republicans.

King's decision will make it more difficult for both parties to determine how to proceed in a race they didn't expect to be a real contest. Snowe, a popular centrist Republican, had been positioned to easily win another term in November, but announced last week she would not run again, opening up a seat that could be winnable for Democrats and even determine whether they hold on to narrow control of the Senate.

But with King as a candidate, the contours of the race are less easily defined. King could spoil Democrats' chances for a pickup by drawing votes away from the Democratic nominee. Or, he could have the same effect on Republicans.

"Which side of the aisle King pulls more votes on depends more than in part in who the two major party candidates are," said Mark Brewer, a political scientist at the University of Maine.

Four Democrats, one independent and one Republican were already in the race when Snowe was believed to be running again, but her announcement set off a scramble by candidates on both sides to determine whether to jump into the race just ahead of the March 15 filing deadline.

Democratic Rep. Mike Michaud (Maine) weighed a run but opted out, while Rep. Chellie Pingree (D-Maine) and former Gov. John Baldacci (D) Are still making up their minds. Republican state Treasurer Bruce Poliquin will run, a GOP source told The Hill, and Maine Secretary of State Charles Summers (R) and state Attorney General William Schneider (R) are also considering.

State Senate President Kevin Raye (R) considered running, but decided to continue in his campaign for Michaud's House seat instead. And Eliot Cutler, who ran for governor in 2010 as an independent, also considered running, but will reportedly back King instead.

"In retrospect, perhaps a little bit of the reluctance of all those candidates to jump in is they really believes — and now they appear to be correct — that Angus was going to get it," Brewer said.

If King wins, he will face the choice of which party's caucus to join. He could choose to caucus with whichever party is more closely aligned with his agenda, and Republicans speculated he would probably caucus with Democrats. Two independents — Sens. Joe Lieberman (Conn.) and Bernie SandersBernie SandersTrump makes his case in North Carolina, Ohio and Wisconsin Trump mocks Joe Biden's drive-in rallies at North Carolina event Sanders hits back at Trump's attack on 'socialized medicine' MORE (Vt.) — currently caucus with the Democrats.

King could also choose to caucus with whichever party wins the majority, therefore aligning himself with the party that will call the shots in the next Senate.

But if the Senate splits in November and King ends up as the 50th vote, his decision about with whom to caucus with could mean the difference between whether Republicans or Democrats control the Senate.