In 2018, 33 US Senate seats, or one-third of the US Senate, will be up for election.

Eight incumbents are Republicans, but each of their states routinely vote Republican and so they are unlikely to face serious competition. At this early date, the following five Democratic incumbents appear to hold the most vulnerable US Senate seats.

#1. Claire McCaskill – Missouri

With Republican candidates winning the state’s races for presidency, governor, and US Senate, the St. Louis Post-Dispatch called the 2016 election an "evolution" of Missouri as a purple state into a bright red one.

McCaskill was a vulnerable incumbent in 2012. Despite Missouri voting comfortably for Mitt Romney, gaffes by her Republican opponent swung the Senate election in McCaskill’s direction. In 2018, Missouri Republicans will likely focus on nominating a skilled candidate to defeat McCaskill.

In a book released last year, McCaskill bragged about her ad campaign designed to benefit her opponent Todd Akin:

"I had successfully manipulated the Republican primary so that in the general election I would face the candidate I was most likely to beat."

McCaskill accomplished this by running an attack ad in disguise -- one that would help Akin with ultra-conservatives, but hurt him among independents.

#2. Bill Nelson – Florida

Nelson first won election to the US Senate in 2000 and was re-elected in 2006 amid a huge Democratic midterm victory.

Although identified as an incumbent Republicans hoped to defeat in 2012, the Democrat still beat US Rep. Connie Mack by more than 10 points.

Nelson has already announced he is running for re-election. He will be 76 years old in 2018.

With Donald Trump winning Florida and Marco Rubio re-elected to the Senate this year, Florida may be a challenging state for Democrats to retain.

#3. Robert Casey, Jr. – Pennsylvania

Casey came into the US Senate after defeating incumbent Rick Santorum in 2006 with one of the largest victories against an incumbent. Casey won again in 2012 with 54% of the vote.

Like his father, a Democratic governor of the state, Casey presents himself as a pro-life Democrat. He officially opposes federal funds for abortion. However, Casey still supports federal funding for Planned Parenthood for its other services. In an op-ed on the subject, he wrote:

"Because of the Hyde amendment, any federal funds going to Planned Parenthood cannot be spent on abortions, but must go to the organization’s other activities."

As with the other senators who may be vulnerable, Casey represents a state Trump won.

#4. Joe Donnelly – Indiana

Donnelly scored one of 2012’s upsets when he won a seat held for six terms by Republican Sen. Richard Lugar.

As in Missouri, a gaffe-prone Republican made headlines for comments on social issues. Also as in Missouri, the state strongly voted Republican for president, but Democratic for the Senate. Consequently, Donnelly defeated Richard Mourdock by 5 points.

Donnelly’s reputation as a moderate may help him in a state like Indiana. However, his movement toward stricter gun control may complicate his position. Donnelly participated in this year’s Democratic filibuster on gun control. He also supported California US Sen. Dianne Feinstein’s measure to allow the Attorney General to reject firearms sales to suspected terrorists.

Donnelly’s vulnerability may be evidenced by the fate of another Indiana Democrat with a reputation as a moderate. Evan Bayh last week lost a bid to re-claim his Senate seat by 10 points.

#5. Joe Manchin – West Virginia

Several have called Manchin among the most conservative Democrat in the Senate. He won a special election in 2010 and a full term in 2012.  So Manchin has shown he can win as a Democrat in election cycles that favor either major party.

Manchin has shown a willingness to criticize and break from his own party. In 2014, he mentioned the president was becoming disconnected from the people. Manchin said, "People just don’t believe he cares." After this year’s election, Manchin called Harry Reid "an absolute embarrassment" over the Democratic leader’s reaction to Donald Trump’s electoral victory.

Manchin is currently denying speculation that he is seeking to switch to the Republican Party ahead of his re-election campaign. He may also choose not to return to the Senate and instead run for governor of West Virginia. Manchin previously served five years as West Virginia's governor.

Each race is unique and electoral factors differ everywhere. In some election cycles, status as a moderate is helpful, but in others it can be a liability. Although minority parties tend to perform well in midterm elections, each of these Democrats represent states Donald Trump won last Tuesday and where Republicans could win again. Consequently, independents will be well-positioned to decide the above five US Senate seats.

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