The final Iowa pre-caucus poll from the highly regarded Bloomberg/Des Moines Register/Selzer poll finds businessman Donald Trump and former Secretary of State Hillary Clinton leading in their parties' contests. In the Republican caucus, Trump leads Texas Sen. Ted Cruz, 28 percent to 23 percent. In the Democratic caucus, Clinton leads Sen. Bernie Sanders (I-Vt.), 45 percent to 42 percent.

Florida Sen. Marco Rubio is trailing in third place among Republicans with 15 percent support and retired neurosurgeon Ben Carson has 10 percent. Other Republican candidates are in single digits.

Notably, former Florida Gov. Jeb Bush, who at $15 million has spent more money on ads than any other candidate in Iowa, has dropped to just 2 percent. At 2012 caucus turnout levels , that works out to about $6,180 spent per supporter.

Polls in recent days have shown a close race for both caucuses.

The HuffPost Pollster average, which aggregates all available polls, shows Trump with a 8-point lead over Cruz with 32 percent of the vote to his 24 percent. Marco Rubio is third at 14 percent. All other candidates have single-digit support.

In the Democratic race, Clinton holds a slim lead over Sanders with 47 percent of the vote to Sanders' 44 percent. Martin O'Malley has 4 percent.

While polls are able to capture a snapshot of the current state of the race, other factors could cause some surprise shifts come caucus day. The Iowa caucuses are particularly fickle due to their format and questions about turnout.

The Des Moines Register/Bloomberg Politics poll, conducted by Selzer & Co. Jan. 26-29, 2016, is based on telephone interviews with 602 registered Iowa voters who say they definitely or probably will attend the 2016 Republican caucuses and 602 registered voters who say they definitely or probably will attend the 2016 Democratic caucuses. The margin of error for Republicans is 4 percentage points, and the margin of error for Democrats is 4 percentage points. Respondents were randomly selected from a list of active voters from the Iowa secretary of states voter registration list. Full poll results can be found here .

Note to our readers: Donald Trump is a serial liar , rampant xenophobe , racist , misogynist, birther and bully who has repeatedly pledged to ban all Muslims -- 1.6 billion members of an entire religion -- from entering the U.S.