9 GOP senators Trump must watch out for
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The fate of President Donald TrumpDonald John TrumpGOP nearing end game on immigration votes Tech companies scramble as sweeping data rules take effect Comey: Trump's 'Spygate' claims are made up 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 McCainJohn Sidney McCainSenate panel advances 6B defense policy bill McCain: Trump pardoning Jack Johnson 'closes a shameful chapter in our nation’s history' Trump pardons late boxing champion Jack Johnson 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 GrahamLindsey Olin GrahamHouse GOP sets three FBI interviews in Clinton probe Trump on collision course with Congress on ZTE The Hill's 12:30 Report — Sponsored by Delta Air Lines — GOP centrists in striking distance of immigration vote 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.

The third senator to watch is Rand PaulRandal (Rand) Howard PaulPro-Trump super PAC raises .5 million in 6 weeks Trump has exposed Democratic hypocrisy on prison reform Overnight Energy: Reporters barred from Day 2 of EPA summit | Dems blame Trump for gas price increases | Massachusetts to get new offshore wind farm 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 CollinsSusan Margaret CollinsOvernight Health Care — Sponsored by PCMA — VA reform bill heads to Trump's desk Senate panel to consider ban on prescription drug 'gag clauses' Pressure rising on GOP after Trump–DOJ fight’s latest turn 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 RubioMarco Antonio RubioHillicon Valley: Experts worry North Korea will retaliate with hacks over summit | FBI works to disrupt Russian botnet | Trump officials look to quell anger over ZTE | Obama makes case for tighter regs on tech Putting pressure on Trump, House passes bill barring government from doing business with ZTE The Hill's Morning Report — Sponsored by PhRMA — Tensions mount for House Republicans 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 FlakeJeffrey (Jeff) Lane FlakeOvernight Energy: Reporters barred from Day 2 of EPA summit | Dems blame Trump for gas price increases | Massachusetts to get new offshore wind farm Jeff Flake: Trump has 'debased' the presidency Senate Democrats look for traction on gas prices MORE of Arizona, an independent-minded reformer and Trump critic; Ben Sasse of Nebraska, a leader in last year's Never Trump movement; Lisa MurkowskiLisa Ann MurkowskiOvernight Health Care — Sponsored by PCMA — VA reform bill heads to Trump's desk Senators introduce bill to measure progress in opioid fight Dems win nail-biter in charity congressional soccer game MORE of Alaska, who called on Trump to resign as presidential nominee after the "Access Hollywood" video surfaced; and Lamar AlexanderAndrew (Lamar) Lamar AlexanderOvernight Health Care — Sponsored by PCMA — Abortion rights group plans M campaign to flip the House Senate health committee to hold hearing on Trump drug pricing plan Overnight Health Care — Sponsored by PCMA — VA reform bill heads to Trump's desk 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.