As the first year of the Trump era nears its close, a tenure thus far characterized by the passing of a disastrous tax overhaul bill and several failed attempts to repeal the Affordable Care Act, the turn of the year has political pundits and eager voters setting their sights on the much anticipated 2018 midterm elections in both the house and the senate.

With a narrow majority in the upper house, the GOP will be looking to expand on their majority, while the Democrats will be looking to win back an advantage they last had in 2014. However, this upcoming round of contested seats will see the latter on the defensive. 23 Democratic Senate seats up for grabs compared to eight for the Republicans, many of which involve states that are among the most competitive to win. Whether its a liberal candidate vying to serve in a predominantly conservative district, or that the riding is dominated by swing voters -- rendering it a political battleground regardless of party affiliation. Here are five senators who will have a tough fight on their hands due to such circumstances:

5. Dean Heller

Elected to the Senate in 2012 to replace retiring incumbent John Ensign, Dean Heller has established himself as one of the most moderate and bipartisan Republicans in the Upper Chamber of Congress. Nevertheless, Heller was won his seat with only 45.87% of Nevadan votes to his opponents 44.71%.

As a Republican Senator in a state that has a record of swinging towards the left, critics and commentators have deemed Heller to be the most vulnerable among his GOP colleagues facing reelection as he gets set to face off against Jacky Rosen, a Democrat currently serving in the House. But before he can worry about her, the native of Castro Valley, Calfifornia will have to find away to squeak past Danny Tarkanian, who leads Heller by six points in the Republican Senatorial Primary.

4. Claire McCaskill

Assuming office in 2007 when the Democrats retook the Senate in the last days of the Bush administration, Claire McCaskill, a moderate-liberal representing the deep red State of Missouri was only expected to last for one term when the 2012 presidential election commenced. Then the improbable happened when Todd Akin, McCaskill's Republican challenger, and the undisputed favourite to win, was bombarded in a storm of controversy when he stated that female victims of rape "rarely get pregnant". This scandal tipped the scales in the Rolla, Missouri native's favour, allowing McCaskill to win by a convincing 54.7% to 39.2% margin.

Nevertheless, another scandal pending her next GOP opponent, McCaskill will find herself in a fight to the last vote when residents in a state Trump won by 18 points take to the polls next fall.

3. Joe Donnelly

Elected to succeed the retiring Richard Lugar in 2012, Joe Donnelly's narrow 50.04% to 44.28% victory helped the Democrats add to their majority in the Senate that had been thinned out in 2010. A supporter of gun rights, an opponent of the DREAM Act, a staunch Pro-Life advocate, while owning a mixed recored on LGBT issues, Donnelly is as conservative as any Democrat is allowed to be.

Nevertheless, he is facing a reelection in a state Donald Trump won by 19 points. With Indiana's history of being a hotbed for the religious right, combined with its traditional preference for candidates bearing the Republican mantle, Donnelly's fight certainly won't be easier the second time around.

2. Heid Heitkamp

Like her colleague from Indiana Joe Donnelly, the Democratic Senator for North Dakota Heidi Heitkamp also possesses conservative sympathies. She possess an A-rating from the National Rifle Association on Gun Rights, supports the Keystone XL Pipeline, and is one of few Democrats who was critical of the Affordable Care Act despite supporting its passage. Despite this, she was just narrowly elected by a 50.24% to 49.32% margin. Now up for reelection in a state that elected Trump by an astounding 36 points, Heitkamp will have a steep hill to climb if she hopes to see the inside of the Senate Chamber for another 6 years.

1. Joe Manchin

In a state that has put forth Robert Byrd and Jay Rockefeller, two of which, are widely regarded among the most influential political figures in recent memory, its hard to imagine that Joe Manchin is all that remains when it comes to Democrats serving in West Virginia -- a state the Republicans won with 68.5% of the vote when they took the Oval Office in 2016. With this mind, Manchin's seat will certainly be one of the Republicans primary targets when the midterm elections role around next fall.

However, the Farmington native, arguably the most conservative delegate amongst his blue leaning colleagues, is extremely popular in his state. Winning his past two Senatorial elections with 53.5% and 60.40% of the vote, respectively. Manchin's reelection campaign also received another boast when Republican Governor Jim Justice announced he would support the incumbent Senator for a third term.