Donald Trump has said that he will boycott the last Republican debate before the Iowa caucuses, prompting Texas senator Ted Cruz to challenge him to a one-on-one debate.
Meanwhile, the White House said President Barack Obama would host Vermont senator Bernie Sanders for an Oval Office meeting, adding intrigue to the Democratic race.
The dual developments created new ripples of uncertainty six days before voting begins in the presidential race.
Both parties are bracing for close contests in Iowa that will determine which of their two candidates will carry the momentum from a victory into the New Hampshire primary and beyond.
On Monday, Iowa residents will gather in schools, churches and even private homes to choose among the Republican and Democratic candidates battling to be their party’s 2016 presidential nominee — the first in a series of state-by-state contests to choose delegates to each party’s presidential nominating convention.
Mr Trump raised the prospect of skipping the debate as he blasted Fox News Channel for “playing games” and including anchor Megyn Kelly as a debate moderator.
Mr Trump’s campaign later said he definitely will not participate.
The race among Democrats was no more settled, with Mr Sanders and Hillary Clinton locked in a close contest and details about their debate plans unclear.
Mr Trump said that he would hold his own event in Iowa during the debate to raise money for wounded veterans.
The billionaire businessman dismissed Ms Kelly as a “third-rate reporter” who is bad at her job and had been “toying” with him — reprising a squabble that erupted after a debate Ms Kelly co-hosted last year.
She shot back on her nightly show, arguing that Mr Trump is used to getting his way but cannot control the media.
She said that her network and CEO Roger Ailes had made it clear to Mr Trump for months that they would not change their line-up.
“I’ll be there,” Ms Kelly said. “The debate will go on with or without Mr Trump.”
The pullout came after Fox News mocked Mr Trump for asking his Twitter followers whether he should debate.
The network, in a sarcastically worded statement, said: “A nefarious source tells us that Trump has his own secret plan to replace the cabinet with his Twitter followers to see if he should even go to those meetings.”
On the Democratic side, as Mr Sanders left Iowa and Minnesota for his meeting with Mr Obama, it was being watched for signs of the president’s leanings.
He has avoided showing favouritism, and White House spokesman Josh Earnest said the meeting would be informal, with “no formal agenda”.
Mr Obama has met Ms Clinton, his former secretary of state and 2008 primary opponent, periodically.