The outcome of the landmark referendum may not be known until the late morning hours of Friday.
A series of polls on Sunday and Monday put 'Unidos Podemos' in second place overall.
Some recent stats have worried pro-EU campaigners, but there's no evidence they're behind.
Trump trails Hillary Clinton by four percentage points in the most recent Reuters/Ipsos poll.
The appearance of the elder Bush on the trail may help Jeb's chances with Republicans in the state.
Trump's lead in New Hampshire has narrowed since Iowa.
The Granite State's first-in-the-nation primary is less than a week away.
Almost 40 percent of those surveyed think Donald Trump would make a "terrible" president.
The three Democratic candidates will have a chance to appear before Iowa voters a week before the state's caucuses.
Mass shootings were the most important story of the year, the poll found.
But it's not clear which candidates will benefit.
The Republican's proposal has been widely criticized and denounced as unconstitutional.
Carson tied for third with Cruz, Rubio moves up to second.