Julian Zelizer is a history and public affairs professor at Princeton University and the author of "The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society." He's also the co-host of the "Politics & Polls" podcast. The opinions expressed in this commentary are his own.

(CNN) This Tuesday, the GOP faces a defining moment. With Donald Trump in the White House and the possibility of Roy Moore in the Senate, the nation is watching closely to see whether the GOP is becoming the party of the GOB -- the good old boys.

While some Senate elections are just about the politics of a state, Moore's victory would leave a huge imprint on the way that voters perceive the Republican Party for years to come. It could easily affect the 2018 and 2020 elections.

Even before the multiple accusations of sexual molestation of minors emerged, Moore (who denies the allegations) was far off the political spectrum. He held extremist positions that would even make some of the readers of Breitbart wince. During a child custody case in 2002, while he was chief justice of the Alabama Supreme Court, he wrote that "homosexual conduct" is "abhorrent, immoral, detestable, a crime against nature, a violation of the laws of nature and of nature's God upon which this nation and our laws are predicated."

He has no respect for the separation between church and state. Moore was removed as chief justice when he refused to dismantle the Ten Commandments monument he had installed in the Alabama Judicial Building. Like President Trump, he was one of the birthers who falsely claimed that President Barack Obama was not born in the United States, and thus his presidency was illegitimate.

In 2006, Moore responded to the election of Keith Ellison, the first Muslim-American elected to Congress, by stating that Ellison shouldn't be able to take his seat.

"Islamic law is simply incompatible with our law," he wrote in an editorial. "In 1943, we would never have allowed a member of Congress to take their oath on Mein Kampf, or someone in the 1950s to swear allegiance to the 'Communist Manifesto.' Congress has the authority and should act to prohibit Ellison from taking the congressional oath today!"

During his campaign, Moore has referred to Native Americans and Asian-Americans as "reds and yellows." And when an African-American at a rally asked when America was "great," Moore responded : "I think it was great at the time when families were united -- even though we had slavery -- they cared for one another. ... Our families were strong, our country had a direction."

Moore even said that 9/11 might have happened because Americans were not religious enough; God was unhappy that "we legitimize sodomy" and "legitimize abortion."

With Trump as President and Moore as a senator, Democrats would have a strong case to make about the GOP jumping deep into the world of identity politics: defining itself through one caricature version of the identity of the white, male, non-college educated voter.

JUST WATCHED Ex-Breitbart spokesman leaves GOP over Moore Replay More Videos ... MUST WATCH Ex-Breitbart spokesman leaves GOP over Moore 01:20

Moore's presence in the Senate would greatly compound the challenge that the party is and will continue to face under President Trump, who has repeatedly connected himself to extremist elements here and abroad, most recently retweeting anti-Islamic videos from the neo-fascist hate group Britain First.

It will become harder for members of the GOP to argue that the party is best suited to lead a nation that has become more pluralistic, more diverse and culturally liberal than ever before. If the Republicans want any moral high ground on cultural questions, it will be much more difficult to claim it with Moore as one of the faces of the party.

And it looks as though Moore is very much in the race despite everything that we have learned. Democratic competitor Doug Jones has only a slim lead over Moore.

Why is it that Moore might pull this off? Why would the Republican Party be taking this kind of risk? It is easy to blame Jones for running a lackluster campaign, but, in fact, he has offered the kind of candidacy and campaign that political analysts have been calling for. He is a white male, a centrist concerned about a strong middle-class economy, a gun-toting Democrat, and at the same time someone with impeccable credentials on civil rights -- as a US attorney, he successfully prosecuted Ku Klux Klan members for a church bombing in 1963.

JUST WATCHED How will Alabama vote in Moore vs. Jones race? Replay More Videos ... MUST WATCH How will Alabama vote in Moore vs. Jones race? 04:44

While Republicans have worked hard to paint Jones as far to the left -- Trump tweeted that he is a "Pelosi/Schumer Puppet" -- he is the kind of middle-of-the-road Democrat who has the best chance of winning in a deeply red state like Alabama.

Though many prominent Republicans have denounced Moore's candidacy -- Sen. Jeff Flake even sent a check to the Jones campaign -- key parts of the Republican Party have fallen behind him. This is essential to why victory is possible. Most importantly, President Trump has now endorsed Moore . "Go get 'em, Roy!" he reportedly said at the end of their phone conversation on Monday morning.

Trump, who said he would not campaign for Moore, held a rally in Florida -- less than 30 miles from the Alabama border -- on Friday. "The future of the country cannot afford to lose a seat in the very, very close United States Senate," Trump said during the rally. "We need somebody in that Senate seat who will vote for our Make America Great Again agenda. So get out and vote for Roy Moore."

Last Monday, the Republican National Committee rejoined Moore's campaign to move him toward victory after cutting ties with him because of the sexual assault accusations. Stephen Bannon, the executive chairman of Breitbart News, who now must be considered a power broker in the party as a result of his work on Trump's campaign and in his administration, has aggressively campaigned for Moore as one of his "anti-establishment" candidates.

The fact is that large portions of the Republican leadership are now comfortable with someone like Moore standing within their tent. It is fallacious to paint him as an outsider, since this campaign proves that the Republican Party has changed and there is sufficient room for these kinds of extremist elements to be welcomed into its ranks.

The deeply-rooted partisan feelings that guide the Alabama electorate are also at play. At some basic level, it seems that many Republican voters in Alabama, including among the sizable evangelical cohort, will vote for a Republican no matter what he or she has done. The candidate could even be accused of child molestation, contradicting the party's professed values, but that is not as important as voting the party ticket.

It is not surprising that the Moore campaign continues to remind voters, in its advertisements, about Moore's strong opposition to abortion -- a key issue for conservatives. The appeal to voters is that all his "peccadillos," as they seem to dismiss them as being, are forgivable if they allow Republicans to retain the seat. As Ron Brownstein has perceptively argued, a Moore victory would come down to his retaining the support of white women who have no college education and who will hold their noses and vote for Moore despite his record with women.

But the short-term gains of retaining the Senate seat outweigh the long-term risks of damaging the image of the party, narrowing its national appeal for presidential elections and taking on damaging baggage with a situation that will likely only get worse. If some Republicans think that they are impenetrable regardless of what the party does, they should take a look at the new Pew Research Center poll showing that President Trump's support is dropping even with his base.

Since February, his support among evangelicals fell from 78% to 61% -- a whopping 17-point decline. His support from non-college educated adults, the heart and soul of the base, down from 56% to 41%.

The Republican Party has reached an important crossroads. And now that Democrats have just pressured one of their most promising senators, Al Franken, to step down over sexual harassment allegations, decision time has come for the GOP. Is it going to be party over principle? It is vital for Republicans to remember that they do have a choice.