President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE is politically more vulnerable than he ever has been, despite the enthusiastic support he enjoys from his (shrinking) base of white, evangelical voters.

His state-by-state job approval numbers suggest that if the 2020 election were held today and he won every state where he has a net positive or tied approval rating (25 states), he would garner 242 electoral votes, 28 votes short of the 270 needed to win the White House. Further, a multitude of criminal investigations surround nearly every aspect of Trump and the organizations he ran with his family.

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To make matters worse, on Jan. 3, Democrats will become the majority in the House of Representatives and, aside from continuing to fight against Trump’s (

largely unpopular

) $5 billion demand to pay for the construction of a southern border wall, they plan to wield their

oversight authority

aggressively. The Trump administration is

poorly prepared

for the volume of congressional requests that soon will arrive to the White House.

Given these facts, it’s hard to imagine that Republican leaders are sanguine about Trump’s pursuit of a second presidential term. Whether the candidate is outgoing Ohio Gov. John Kasich, outgoing Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE of Arizona, or Sen. Ben Sasse Benjamin (Ben) Eric SasseMcEnany says Trump will accept result of 'free and fair election' McConnell pushes back on Trump: 'There will be an orderly transition' Trump says he'll sign order aimed at protecting premature babies in appeal to religious voters MORE of Nebraska, rumblings about serious nomination challenges are emanating from many Republican quarters.

Still, as Washington Post columnist Michael Gerson recently noted, “Toppling a sitting president of your own party is a maneuver with the highest degree of difficulty.” While Gerson is right in describing the challenge, he goes on to argue that the “most relevant historical model is probably Eugene McCarthy’s race against President Lyndon B. Johnson in 1968.” But this isn’t quite true.

From the modern era, the better analogy is 1980, and the fight that was waged by Sen. Ted Kennedy (D-Mass.) against President Jimmy Carter for the Democratic nomination. Like Trump, Carter in 1976 had surprised his party and many of the presidential “favorites” then serving in the Senate by winning the nomination as an “outsider.” Also, like Trump, when Carter won the White House, he had a difficult time working with those in his party who held the majority in Congress. Many of his fellow partisans disagreed with his fiscally conservative approach, which they saw as contributing to the faltering economy. In November 1979, shortly after the Iranian hostage crisis began, Kennedy officially jumped into the Democratic nomination race, touting the need for “new leadership.”

Though Kennedy made several missteps, including a rambling television interview where he failed to explain why he was running, he managed to win pledged delegates across the country and to lead a ferocious effort to upset the nomination balloting at the Democratic National Convention. But Carter had been prepared for the challenge from Kennedy — and from California Gov. Jerry Brown. For months, Carter’s team had worked to burnish his policy accomplishments, elevate his position as the incumbent, and lay down the tracks for a state-by-state primary campaign, which included raising millions of dollars and hiring field staff.

This historical precedent reminds us of two things that are not true today when we consider Trump and the Republicans who may mount a nomination campaign against him. First, Carter’s primary opponents were viewed more as “show horses” than “work horses.” Brown and Kennedy both were perceived as shallow and overly confident media hounds. Many of the names being floated as potential challengers to Trump are the opposite. They are more substantive, experienced and discerning — all-around more morally-grounded men than Trump. Second, Carter could be both a savvy and disciplined candidate when it was required. Trump can’t seem to stop either his bombast or his lying.

Whether Trump would lose his party’s nomination remains unclear because today’s political polarization has fostered such a blind devotion to the leaders of one’s party. Still, with Trump’s latest approval rating again below 40 percent, his weakness seems to be increasing, rather than abating, and a nomination challenge seems in the offing.

As former Rep. Mo Udall (D-Ariz.) cogently explained in relation to the possibility with Carter:

“There are only two conditions when a party challenges its own incumbent president. One is where major elements of the party feel so intensely and so bitterly about one issue that they oppose him no matter what the consequences. They don't care if it costs the presidency. That’s what happened to Lyndon Johnson in 1966 over Vietnam. The other time is when their own president looks so weak, runs so persistently and significantly behind opposition candidates in the polls, that others in the party begin fearing he’s a loser and they’ll lose, too — if they stick with him.”

The ball hasn’t yet dropped in Times Square, but already 2019 is shaping up to look like 1979. Happy New Year, Mr. Trump.

Lara M. Brown, Ph.D., is an associate professor and director of the Graduate School of Political Management at the George Washington University, and formerly was an assistant professor of political science at Villanova University. She frequently appears on TV and radio programs as an expert on American political history, party development and national elections. Follow her on Twitter @LaraMBrownPhD.