When the 2012 battle for the White House grinds to a halt Tuesday, it will signal the end of one of the most contentious and most expensive contests for the presidency the nation has ever experienced.

A campaign that started 18 months ago as a drawn-out, bitter race for the Republican nomination turned into an intense and sometimes personal fight between Democratic President Obama and GOP nominee Mitt Romney.

Each candidate experienced moments of optimism when the rhythms of the campaign turned momentum his way. And each suffered gut-wrenching setbacks, sometimes self-inflicted, as when a Romney video surfaced in which he talked about the 47 percent of Americans dependent on the government, or when Obama appeared surly and disengaged during the first debate.

Political experts say that opening debate on Oct. 3 in Denver may prove to be the most memorable and important event of the contest. Romney was the clear winner of the encounter, and his campaign, foundering throughout September, was suddenly energized.

Nearly 70 million people viewed Obama's listless performance. In the aftermath, Romney rose to a statistical tie in the polls and reversed the Obama campaign's half-billion dollar advertising effort to define him as a cold corporate CEO with little interest in helping the middle class.

"No single event or moment or story was more important than that first debate. It changed the entire race," Republican strategist Matt Mackowiak told The Washington Examiner.

Obama did better in the second and third debates, winning technical victories on the scorecards of most pundits. But by then, the race had been redefined.

The two candidates spent October crossing a handful of swing states, looking for an Electoral College edge in places like Norfolk, Va., Lima, Ohio, and Cedar Rapids, Iowa.

Obama visited Florida and Wisconsin on the campaign's final full day while Romney greeted big crowds in Virginia before heading back to Ohio for a last-minute rally on Tuesday.

After months of sweat, toil and perhaps late-night tears, neither candidate pulled away. The Real Clear Politics national polling average shows the candidates are deadlocked.

Campaign cash from both the candidates and supporting super-PACs played a starring role in both the primary and the general elections and this contest is likely to break spending records, with each campaign topping the billion-dollar mark, the first time any candidate had done so in American history.

And that was only a portion of the money showered on shell-shocked battleground voters who turned on their televisions in recent weeks.

In the general election cycle, the campaigns and supporting super-PACs spent billions on Obama and Romney, much of it in the battleground states of Ohio, Virginia, Iowa, Colorado, Florida and Wisconsin. The final cost, including money spent in the primary, has yet to be calculated. But the Center for Responsive Politics predicts it will reach $6 billion.

Obama's financial advantage may go down as the reason behind a Romney defeat. Obama spent millions in early summer for ads portraying Romney negatively in key battleground states like Ohio even before Romney officially became his party's nominee.

"If Romney loses," Mackowiak said, "it will be because Obama succeeded at defining him earlier in the summer."