The holiday season is upon us. Which means that while Americans are recovering from an orgy of overeating and Black Friday shopping, the political world is easing into the next phase of the presidential election: primary crunchtime.

The period between Thanksgiving and the first presidential primaries and caucuses in January and February is typically full of flux and ferment. The contenders sharpen their messages. The campaigns flood Iowa, New Hampshire and other early-going states with additional money and people and ads. So many ads. More voters start paying attention. Watching candidates surge and fizzle, focus and fold, you often can get a sense of how they respond under pressure. And if there’s one thing a president needs to be able to handle, it’s pressure.

In a normal election, these early contests can bring all kinds of surprises. In 2000, John McCain’s maverick run upset George W. Bush in the New Hampshire primary, jolting the Republican nomination race. In 2004, during the weeks leading up to the Iowa caucuses that January, a floundering John Kerry loosened up, warmed up and crisped up his message (“The real deal”!) in the Democratic race, crushing the dreams of the anti-establishment darling Howard Dean. (Remember the Dean scream? Good times.) In the 2008 election, Team Obama started working Iowa early and just kept turning the heat up as caucus night approached, driving a stake through Hillary Clinton’s aura of inevitability. And so on.

This time, obviously, the state of play is different. With Donald Trump, the Republican contest includes a de facto incumbent whose dominance looks all but insurmountable. Some players have already left the field. Others need to leave A.S.A.P. (Looking at you, Asa and Doug.)

But this race is not over. In fact, not a single vote has been cast. And for all Mr. Trump’s advantages, he’s lugging around some heavy baggage that gives the primary a tremor of instability.