We ve known for more than a year that some Republicans are less than thrilled with Donald Trump, and now they re even more irritated because he won and they get to put up with him. Now the party must grapple with his extreme agenda, and there are actually several Republican senators who are looking to block, or at least mitigate, some of what Trump wants to do.These aren t just any Republican senators, either. Many have a lot of influence and are seen as leaders, whether they re officially part of the leadership or not. They include Sens. Susan Collins (Maine), Jeff Flake and John McCain (Arizona), Rand Paul (Kentucky), Lamar Alexander (Tennessee), Ben Sasse (Nebraska) and Lindsey Graham (South Carolina).And all of them have different agenda items against which they ll work.One of the worst potential thorns in Trump s side could be Rand Paul. In 2013, he was part of the infuriating group that shut down the government for 16 days over the Affordable Care Act, but later said it was a  dumb idea.  He s been a terrible obstructionist, but he is willing and ready to use his obstructionism to filibuster Trump s cabinet picks.Yes, he will turn against his own to prevent some of Trump s cabinet nominations from getting confirmed if he has to. He spoke out against both John Bolton and Rudy Giuliani for Secretary of State, going so far as to call Bolton  a menace.  Bolton doesn t seem to be in the running anymore, but Giuliani is. There s a possibility that Paul might oppose other potential picks as well, depending on who they are and how Paul sees them.It would only take three Republicans to block Trump s nominees on the floor, assuming Democrats unite against any controversial picks.Susan Collins of Maine is more centrist than many of her Republican colleagues, and she s very good at building bipartisan coalitions, along with others in this group. If need be, they can gain control over an issue and take some of the wind out of the leadership s sails.Ben Sasse of Nebraska is prepared to challenge Trump on executive overreach. Trump has said, in plain language, that he plans to use executive orders to accomplish some of his agenda on his very first day in office. He also used words like  reign,  during his campaign, which Sasse took major issue with. Depending on what all those items are, Trump could feel pushback from Sasse and other conservatives looking to rein in the executive branch.John McCain carries a deep distrust of Trump s buddy Putin. He, Lindsey Graham and even Ben Sasse don t believe it s a smart idea for us to try and work more closely with Russia, especially if it comes at the expense of our European allies. That could mean opposing any effort to withdraw from NATO and opposing joining forces with Russia and Syrian dictator Bashar al-Assad.We ve tried to  reset  relations with Russia before, and Putin decided to play conqueror and invade Ukraine. McCain rightfully believes Putin is not to be trusted. Considering his and Trump s bromance, Trump may not be trustworthy in this arena either, making McCain and Graham into major problems for him.Lamar Alexander has been urging his colleagues to proceed cautiously with trying to repeal the ACA because developing a replacement could take years. Simply pulling it out  root and branch  as quickly as possible could be disastrous for the U.S., particularly if it creates a vacuum while they re hashing out every idea imaginable for replacing it. It would especially hurt the working class, many of whom have been able to get healthcare thanks to expanded Medicaid coverage and who Trump claimed to represent.Alexander also has close relationships with Democratic Senators Chuck Schumer (the new minority leader), and Patty Murray. Patty Murray is the senior Democrat on the Senate Health, Education, Labor and Pensions Committee, which Alexander chairs. He can be a voice of reason sometimes, if for no other reason than he really doesn t like feeling like he must vote along party lines.It s true that some of these issues will put these senators at odds with each other. However, the Senate is split 51-48 right now, which could go to 51-49 depending on what happens in Louisiana next month. The narrow margin means it doesn t take many to create a pseudo-majority of Democrats in this environment.And while, yes, every one of these senators has been problematic for progress over the years, any Republican opposition to Trump could be extremely helpful.Every one of these senators has said that they ll take things issue by issue, rather than oppose Trump unilaterally. It s also hard to say just how committed to their issues they are vs. how committed to toeing the party line they are. If they actually remain true to their consciences, they ll be the senators we need to keep checks on the more extreme House, and on Trump s own insanity.Featured image by Drew Angerer via Getty Images