The fate of President Donald Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE's legislative agenda will be determined in the Senate.

Even though Republicans control the body, their 52-seat majority is thin and can't stand many desertions. Each Republican senator is critically important. It will only take a few of them to stop the Trump train on any given issue.

The Senate's 60-vote majority is always a stumbling block on matters where filibusters can be used. Even on issues where it only takes a simple majority to get things done in the Senate — such as confirming presidential appointments (other than the Supreme Court) and passing reconciliation legislation — getting the needed 51 votes may, in some instances, be difficult.

With this in mind, it’s worth watching nine Republican senators who, for one reason or another, could play a big role — including that of adversary — on key parts of the new White House agenda.

The most prominent GOP senator who spells trouble for Trump is John McCain John Sidney McCainKelly's lead widens to 10 points in Arizona Senate race: poll COVID response shows a way forward on private gun sale checks Trump pulls into must-win Arizona trailing in polls MORE. The former presidential nominee gets loads of media coverage, especially when he bucks his own party. A former prisoner of war in Vietnam, he will never forget Trump's criticism of his military service ("I like people who weren't captured.") The irascible Arizona senator is already on the warpath, focused on U.S. relations with Russia and the hacking issue.

Another thorn in Trump's side is Lindsey Graham Lindsey Olin GrahamSenate Republicans signal openness to working with Biden Loeffler calls for hearing in wake of Netflix's 'Cuties' Quinnipiac poll shows Graham, Harrison tied in South Carolina Senate race MORE of South Carolina. He ran against Trump for the GOP nomination and said some rough things on the campaign trail. Graham and McCain are working in tandem — as they usually do — to box in Trump on national security issues.

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The third senator to watch is Rand Paul Randal (Rand) Howard PaulGOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill Overnight Health Care: Senate Democrats block GOP relief bill | Democrats reveal Medicaid chief's spending on high-paid consultants | Trump calls question about why he 'lied' about COVID-19 a 'disgrace' MORE, also a Trump rival in last year's presidential primaries. The two wrangled throughout the presidential debates, and Trump even made fun of the Kentucky senator's appearance. Paul is already questioning Trump's approach on healthcare reform and its impact on the federal deficit.

Then there is Susan Collins Susan Margaret CollinsThe Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally Gideon leads Collins by 12 points in Maine Senate race: poll Senate leaders quash talk of rank-and-file COVID-19 deal MORE of Maine, perhaps the least conservative Republican in the Senate. She's remembered for her famous "I will not be voting for Donald Trump" statement after he became her party's presidential nominee.

Next is Marco Rubio Marco Antonio RubioHillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Intelligence chief says Congress will get some in-person election security briefings Republican Senators raise concerns over Oracle-TikTok deal MORE, who famously squared off against the New York billionaire during their nasty GOP nomination brawl. The Florida senator, who still harbors higher ambitions, may want to keep a respectable distance from his former nemesis on some issues. His tough questioning of Secretary of State nominee Rex Tillerson was an early example.

There is also Jeff Flake Jeffrey (Jeff) Lane FlakeRepublican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style Bush endorsing Biden? Don't hold your breath MORE of Arizona, an independent-minded reformer and Trump critic; Ben Sasse of Nebraska, a leader in last year's Never Trump movement; Lisa Murkowski Lisa Ann MurkowskiGOP ramps up attacks on Democrats over talk of nixing filibuster OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' House Democrats tee up vote on climate-focused energy bill next week MORE of Alaska, who called on Trump to resign as presidential nominee after the "Access Hollywood" video surfaced; and Lamar Alexander Andrew (Lamar) Lamar AlexanderTrump health officials grilled over reports of politics in COVID-19 response Now is the time to renew our focus on students and their futures CDC says asymptomatic people don't need testing, draws criticism from experts MORE — a former governor of Tennessee, U.S. Education secretary and presidential candidate — who has tussled with Trump over the years.

Of course, Republicans in Congress who too often oppose their president must be prepared to hear from his supporters back home and maybe even risk a primary challenge in the next election. Flake already has a Trump backer running against him in the 2018 GOP primary.

For sure, Trump will have his hands full fighting off Democrats. They've already set up a war room to oppose him. But, the new president will also have to look over his shoulder at some of his fellow Republicans.

Yes, Trump is captain of the ship — but it will never leave the dock unless most of the Gang of Nine are on board.

Ron Faucheux, Ph.D., is a political analyst, author and pollster. He publishes LunchtimePolitics.com, a daily newsletter on polls. He also runs Clarus Research Group, a nonpartisan survey research firm.

The views expressed by contributors are their own and not the views of The Hill.