BOSTON (AP) — Massachusetts voters weary over the state's seemingly endless succession of elections be warned: the parade of candidates isn't ending any time soon.

The state's third Senate election in four years has just wrapped up, but attention is already turning to a slew of upcoming races.

Candidates are lining up for the next special election — this one to fill the congressional seat of newly minted Sen.-elect Edward Markey. And in Boston more than a dozen would-be successors to Mayor Thomas Menino are spending the summer courting voters.

But the biggest contest on the horizon is next year's governor's race.

Neither Gov. Deval Patrick nor former Lt. Gov. Tim Murray is running, leaving no clear Democratic front-runner.

Attorney General Martha Coakley, state Treasurer Steven Grossman, U.S. Reps. Michael Capuano and Stephen Lynch, Somerville Mayor Joseph Curtatone, and state Sen. Daniel Wolf of Harwich are among the Democrats considering running.

Two lesser-known Democrats — Donald Berwick, a former top health care official in President Barack Obama's administration, and Joseph Avellone, a life sciences company executive and former Wellesley selectmen — have already announced their campaigns.

There's an equally open field on the Republican side, although two possible candidates are getting the most attention — former gubernatorial candidate and one-time CEO of Harvard Pilgrim Health Care Charles Baker and former U.S. Sen. Scott Brown.

Democrats could get an early look at their potential field next Saturday, when the party holds its annual convention in Lowell.

Massachusetts Democratic Party Chairman John Walsh said that while only declared candidates can address the more than 3,000 expected party faithful, undeclared hopefuls are also expected to attend to try to scope out their chances.

"It's an opportunity for the candidates who are already running to make an introduction to most of the people who will be delegates next year," Walsh said. "I expect all of the candidates will be working the floor."

In 2005, a little-known former Justice Department official named Deval Patrick used the convention to garner backing for his long-shot 2006 campaign.

At the moment all eyes are on Coakley, who has a strong statewide political organization and, despite her loss to Brown in the state's special 2010 U.S. Senate race, remains popular with voters.

Coakley is also facing one of the most unusual political hurdles in Massachusetts political history — the curse of the attorney general's office.

Since 1958, five former Massachusetts attorneys general have sought the governor's office. All five — George Fingold, Edward J. McCormack Jr., Francis X. Bellotti, Scott Harshbarger and Tom Reilly — have failed, either by losing their party's primary, losing the general election, or in the case of Fingold, dying before Election Day.

The most recent spate of political churning began with the death of longtime U.S. Sen. Edward Kennedy in 2009.

Since Kennedy's death, the state has gone through a 2010 special U.S. Senate election to fill the rest of his term, a 2012 election for the full six-year term, and the just-completed 2013 special election to fill the remainder of John Kerry's term after he left the Senate to become secretary of state.

Those hoping to fill Markey's congressional seat are already scrambling to round up votes.

There are five Democratic hopefuls: Middlesex Sheriff Peter Koutoujian, state Sen. Karen Spilka of Framingham, state Rep. Carl Sciortino of Medford, state Sen. Katherine Clark of Melrose and state Sen. William Brownsberger of Belmont.

One Republican, attorney Frank Addivinola, has also jumped in the race.

Under state law, the special election will be held within 160 days of Markey formally resigning after being sworn-in to the Senate seat, which could happen this week.

And there are plenty of other political contests in the offing.

Anyone hoping to run for the state's other constitutional offices — including attorney general, secretary of state, auditor, treasurer and lieutenant governor — have to begin putting together a campaign organization soon.

Every seat in the Massachusetts House and Senate is also up for grabs next year.

And about that just-completed Senate race?

Markey will face election again next year to the full six-year term, giving Republicans and other Democrats yet another shot at a Senate seat in what will be the state's fourth U.S. Senate race in as many years.

Some of those eyeing the governor's office have already begun stockpiling cash.

Grossman has the early fundraising lead, with $565,158 in his account at the end of June. In June alone, Grossman's collected more than $139,000 — easily his most successful fundraising month this year.

Coakley came in second to Grossman. As of the end of June, she had a balance of $257,669.

Berwick has also been busy gathering money. In the first six months of the year, he's accumulated an account balance of $210,032. Avellone, who also opened his account at the beginning of the year, reported a balance of $60,800 as of the end of June.

As of the end of 2012, Wolf had $25,747 in his account while Curtatone had $139,500.

Members of Congress, like Capuano or Lynch, are barred from transferring money from their federal accounts into a state account.

Baker is also no fundraising slouch. During the 2010 governor's race, he raised more than $7 million.