With seven days until Iowans take the first official step in determining the fate of 2016's presidential hopefuls, Donald Trump and Hillary Clinton are holding their front-runner edge as rivals work to score last-minute support that could thrust them to the front -- or, keep them alive.

On the Republican side, the latest Fox News Poll shows Donald Trump regaining his lead in Iowa and surging past Ted Cruz. Looking to make up lost ground, the Texas senator was barnstorming the Hawkeye State on Monday, and announced the endorsement of former candidate and ex-Texas Gov. Rick Perry.

On the Democratic side, Clinton and Vermont Sen. Bernie Sanders likewise are in a tight race for caucus-goers. But given Sanders’ big lead in New Hampshire – which holds the first primary eight days later – Iowa arguably is more important for her than for Trump, who leads decisively in the Granite State.  

To that end, the wide field on the Republican side means success in Iowa and New Hampshire will be determined not only by who finishes first -- but who can make it into the top three.  

In Iowa, Florida Sen. Marco Rubio appears to be in solid third place position in most polls.

Fox News analyst and former George W. Bush senior adviser Karl Rove said Monday that his recent endorsement by the Des Moines Register should also help, among voters who might be inclined to support him.

“That’s a big plus,” Rove told Fox News. In New Hampshire, the battle for second position is even more intense – with Cruz, Rubio, Ohio Gov. John Kasich, New Jersey Gov. Chris Christie and former Florida Gov. Jeb Bush also tangled up in the polls. Rove said it’s possible any one of them could jump into second place there.

But a lot could still rest on who finishes first in Iowa.  

Rove said if Trump notches a “strong victory” in the first two contests, and goes on to do well in South Carolina and Nevada after that, “it’s going to be hard for anybody to knock off somebody who has demonstrated, in the first four contests, a consistent win.”  

Trump’s candidacy has exposed a fissure in the so-called Republican establishment, with some party figures and senior leaders praising him and others outright lambasting his candidacy.

The conservative National Review put out a special issue last Friday purporting to state the conservative case against the billionaire businessman whose political bravado – more than any conservative orthodoxy – has characterized his campaign and attracted supporters. The magazine’s editorial called Trump a “menace to American conservatism,” a message echoed by essays from other conservative contributors.

Trump responded by calling the magazine a “failing publication.”

But not all in the so-called “establishment” are distancing themselves from Trump.

In Iowa, senior Sen. Chuck Grassley, a Republican, introduced him at a rally over the weekend, while stopping short of an official endorsement. And Iowa Republican Gov. Terry Branstad has been openly urging caucus-goers not to support Cruz, in part because of his stance on ethanol.

But Cruz, who until recently was trading the lead with Trump in Iowa polls, is campaigning hard for the Iowa vote. On Monday morning, fellow Texan Rick Perry got behind Cruz’ candidacy, calling him a “very committed conservative” who’s ready to lead on “day one.”

His is hardly the only late endorsement before the first two contests. Former Minnesota Sen. Norm Coleman, speaking on Fox News, endorsed Bush for president on Monday.

And last week, Trump snagged a high-profile endorsement from former Alaska Gov. Sarah Palin.

Meanwhile, it remains to be seen whether Iowa’s results will be a real indicator of the race months from now.

The state in recent cycles has had a mixed track record of picking winners. On the Republican side, Iowa has picked the eventual nominee in two of the last four contested caucuses going back to 1996. The last two winners – Mike Huckabee and Rick Santorum – did not become the nominee. On the Democratic side, the state has a better record; not since 1992, when it chose Tom Harkin, has Iowa picked a losing political horse.

Right now, Clinton is edging Sanders in the Democratic race, in the RealClearPolitics average of polls.

But Sanders, speaking Monday at a community college in Iowa Falls, defended his electability in a general election – trying put at ease those voters who might not think he can go up against someone like Trump. He said that when someone says they don’t want to vote for Sanders because he can’t beat Trump, Sanders said, “That’s just not accurate.”

Clinton, Sanders and former Maryland Gov. Martin O’Malley plan to participate in a CNN-hosted town hall Monday night.