9 GOP senators Trump must watch out for
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The fate of President Donald TrumpDonald John TrumpWarren: 'White supremacists pose a threat to the United States like any other terrorist group' National Enquirer paid 0,000 for Bezos texts: report Santorum: Trump should 'send emails to a therapist' instead of tweeting 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 McCainSantorum: Trump should 'send emails to a therapist' instead of tweeting Meghan McCain: Trump obsessed with my father because he 'will never be a great man' CNN's Amanda Carpenter: Trump attacking McCain 'to distract' from 'questions about the Russia investigation' 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 GrahamOverwhelming majority of voters want final Mueller report released: poll Bottom Line Pence traveling to SC for Graham reelection launch 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 PaulTrio of NFL players intern on Capitol Hill as part of league program Trump keeps tight grip on GOP GOP moves to rein in president's emergency powers 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: CDC pushes for expanding HIV testing, treatment | Dem group launches ads attacking Trump on Medicare, Medicaid cuts | Hospitals, insurers spar over surprise bills | O'Rourke under pressure from left on Medicare for all Dem group launches ads attacking Trump's 'hypocrisy on Medicare and Medicaid cuts' Juan Williams: Don't rule out impeaching Trump 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 RubioDem senator wants Trump to extend immigration protections to Venezuelans Juan Williams: Don't rule out impeaching Trump Trump keeps tight grip on GOP 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 FlakeTrump keeps tight grip on GOP McSally to back Trump on emergency declaration Flake: Biden 'strikes fear in a lot of Republicans' 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 MurkowskiJuan Williams: Don't rule out impeaching Trump The 25 Republicans who defied Trump on emergency declaration Overnight Defense: Senate rejects border emergency in rebuke to Trump | Acting Pentagon chief grilled on wall funding | Warren confronts chief over war fund budget MORE of Alaska, who called on Trump to resign as presidential nominee after the "Access Hollywood" video surfaced; and Lamar AlexanderAndrew (Lamar) Lamar AlexanderWhite House proposes limits on student loan borrowing as part of higher education reforms The 25 Republicans who defied Trump on emergency declaration Trump issues first veto, warning of 'reckless' resolution 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.