WASHINGTON (AP) — The U.S. presidential election is still over 17 months away, but millions of dollars are already flowing into political war chests in what may be America's most costly contest for the White House.

Campaigns are already grinding toward full throttle with more than seven months before the first of the state-by-state primary elections and caucuses and the hectic pace is not likely to let up before next summer's national party conventions.

What explains the extended U.S. campaign season?

In the parliamentary systems that govern most developed countries, sitting legislators representing the political parties choose their leader. That person becomes prime minister. National laws limit campaigning to just weeks or a few months.

Not so in the American system.

"For foreigners it's all strange. They think that everything is money in America. It's all about campaign finance and that people are being bought. And, of course, sometimes that happens. But it's much more complex than that," said James Thurber, director of American University's Center for Congressional and Presidential Studies.

While candidates might prefer to hold back the start of campaigns, none of them can take the chance of being left behind. Once one candidate starts the race, others know they have to join in.

Former-Florida Gov. Jeb Bush, the brother of one president and son of another, declared his interest in running in December, propelling other Republicans to start their campaigns.

But Bush only officially joined the race last week, waiting until 10 other candidates had announced, to avoid fund-raising strictures that apply to declared candidates as opposed to those who are just considering a run. In the latter case, collecting money as a possible candidate is restrained only by how much donors are willing to chip in.

The Democrats are fielding a more modest field, led by Hillary Clinton, the former secretary of state, New York Senator and first lady to Bill Clinton. Vermont Sen. Bernie Sanders, an independent who votes with the Democrats, has enjoyed a surprisingly fast start against Clinton, drawing big crowds to campaign events.

But Clinton commands a formidable lead in fund raising and is still the prohibitive Democratic favorite for the nomination in a contest that also includes former Maryland Gov. Martin O'Malley and Rhode Island Gov. Lincoln Chafee.

"There's a whole lot of money that has nothing to do with (federal) campaign finance (laws) that is being used in coalition building, grass roots, top roots, advertising, social media." Thurber said

That's especially so in money spent on get-out-the-vote cadres, who can lead to victory in close elections. The percentage of eligible Americans who cast ballots in the 2012 election that returned President Barack Obama to the White House was just 58.2 percent.